STIs are infections that are transmitted through close body contact, usually sexual. Sexual contact includes fucking, sucking, as well as touching, fingering or kissing/licking a cock, arse or vagina.
STIs can be caused by viruses (e.g. HIV, herpes, genital warts), bacteria (e.g. chlamydia, gonorrhoea), or parasites (e.g. crabs).
Many STI's are easily transmitted even when a condom is used and you will not always have symptoms, so it is very important to test regularly.
The recommendation for Gay and Bi guys is to have a full STI test at least once a year - every 3 months if you have multiple sex partners or casual partners.