STI stands for sexually transmitted infection. We used to call them STDs (sexually transmitted diseases) but we don’t anymore because when we get an infection through sexual contact, we don't always experience symptoms and the infection may not develop into a disease. An example of an infection turning into a disease would be a chlamydia infection that is left untreated and turns into a Pelvic Inflammatory Disease. That's the reason we now use the term STI instead of STD.
You might also see the acronym STBBI. STBBI is when we combine the shorthand for “sexually transmitted infection” and the shorthand for “blood borne infection,” which refers to infections that can pass from one person to another through blood and other body fluids during sexual contact.
These are all different terms for the same thing: infections we can get through sexual contact (sex of many kinds) with a partner who is infected. STIs are quite contagious and a common experience, though many of us don’t have symptoms of an infection even if we have one. That’s why it’s important to get tested at least once a year.