What an internal condom is and how it works
An internal condom is a barrier device worn inside the body during sex to reduce the risk of pregnancy and sexually transmitted infections. The NHS describes internal condoms (sometimes called female condoms) as helping prevent pregnancy and reduce the risk of STIs (NHS: Internal condoms). Wikipedia describes the same device as a barrier contraceptive used during intercourse to reduce the probability of pregnancy or STI (Wikipedia: Female condom).
The mechanism is physical containment: the condom is inserted into the vagina or anus before intercourse to reduce exposure to semen or other body fluids (Wikipedia: Female condom). A ring or foam disc at the closed end is used to insert the condom and to hold it in place during intercourse (Wikipedia: Female condom). That is the whole idea — a sheath that lines the passage and catches fluid rather than letting it contact tissue.
How effective is it?
The effectiveness figure you should carry is the typical-use number, not the perfect-use number. Wikipedia reports that the female condom is a barrier contraceptive used to reduce the probability of pregnancy or STI (Wikipedia: Female condom), and the NHS frames internal condoms as a method that helps prevent pregnancy and reduce STI risk (NHS: Internal condoms).
What changes the answer is consistency. A barrier only reduces risk when it is used correctly and every time. The sources describe the device as reducing probability rather than eliminating it. Treat the figure as a risk-reduction estimate rather than a guarantee. If you need a precise typical-use percentage for your own situation, confirm it on the NHS page before relying on a number.
How to use one: the practical routine
The routine is insert, check placement, then remove after sex. Insert the condom into the vagina or anus before intercourse so it reduces exposure to semen or other body fluids (Wikipedia: Female condom). Use the inner ring or foam disc at the closed end to push it in and to hold it in place during intercourse (Wikipedia: Female condom).
The condition that changes the routine is placement: the closed end goes in first and stays anchored, while the open end remains outside the body. If the ring or disc is not holding position, stop and reposition before continuing rather than proceeding with a condom that has slipped. After sex, remove it before standing up where possible, and dispose of it rather than reusing it.
Side effects and what to expect
Most of what you will notice is mechanical rather than pharmacological. Because the device is a physical barrier inserted into the vagina or anus (Wikipedia: Female condom), the realistic expectations are awareness of the ring or disc, possible discomfort on insertion, and the need to check placement during use.
The condition that changes this is fit and lubrication. A condom that sits correctly and is adequately lubricated is less likely to cause friction discomfort. One that is poorly placed is more likely to slip or feel intrusive. If insertion is painful or the device will not stay in place, stop and treat that as a signal to change approach rather than pushing through.
Who it suits
Internal condoms suit people who want a barrier method they control themselves, and they are an option for vaginal or anal sex. The device is inserted into the vagina or anus before intercourse to reduce exposure to semen or other body fluids (Wikipedia: Female condom). Makes it usable by the receptive partner rather than depending on the insertive partner.
What changes the fit decision is anatomy and preference. Someone who finds the ring or disc uncomfortable, or who cannot reliably place it, may do better with another barrier method. Someone who wants a method that does not require their partner to wear anything has a reason to try it. The honest boundary is that the sources describe the device and its purpose. They do not tell you which method is right for your body. Use the description to judge fit and confirm details on the NHS page.
If something goes wrong
If the condom slips, breaks, or is not in place during sex, treat it as a failure of the barrier and act on that basis. The device works by reducing exposure to semen or other body fluids (Wikipedia: Female condom). A displaced or damaged condom means that reduction did not happen as intended.
The fallback step is to stop, remove the condom, and seek advice on emergency contraception and STI testing rather than waiting. The NHS internal condoms page is the destination to confirm the current guidance for your situation (NHS: Internal condoms). Do not reuse a condom that has slipped or been removed.
A practical safety note: this page describes the device and its routine. It does not replace advice from a clinician about your own risk, and if you are unsure after a failure, contact a sexual health service promptly.