An STI is a sexually transmitted infection. You can get an STI by having sexual intercourse or sexual contact with another person who is infected. Getting an STI can be frightening, embarrassing, and can be dangerous especially if left untreated. It can even be life threatening. What is worse is that some people who have an STI don't even know it.
That is why it is important to play it safe. The best way to protect yourself from an STI is to practice abstinence-in other words no sexual contact. However that's not a reality for most people. Contraceptives and condoms may also be a way to protect yourself from some STI's, but it won't protect you from all of them. Regular sexual health screening at a Sexual Health Clinic is recommended at least annually or every time you have a new partner.
Remember that some STI's can be easily treated and cured if you're diagnosed early. So don't be embarrassed about getting tested for an STI. It's worth it.