STIs (they used to be called STDs or sexually transmitted diseases) are infections that can be spread during sexual contact (vaginal, anal, oral sex). Some infections like herpes and warts can also be transmitted by skin to skin contact.
STIs are scary things because most of them have no symptoms - so you don't know you have them. Young people are especially at risk of an STI called chlamydia. If chlamydia is left untreated, it could leave you infertile (so you can't have a baby). Most STIs are easily treated so it's important to have an STI test if you have had unprotected sex. STI tests are quick and painless and can be free of cost.
Remember that condoms and water-based lubricant are the only form of contraception that will protect you from both STIs and unplanned pregnancy.