A femidom is a female condom, i.e. a contraceptive. It also protects against sexually transmitted diseases.
It is approximately 18 cm long and has rings at both ends. One of these lies outside the woman's yoni, the other is inserted and is of course closed at the end. The protection against unwanted pregnancy is not as high as with male condoms.
It can be deployed hours before a union. Femidom has been around since 1990 and is constantly being improved. Sometimes the outer ring is significantly larger and more triangular than round.
You can get more information on the Internet or in the pharmacy. Price per piece could be around 3-5€.