Chlamydia is an infection that you can get if you have sex without a condom and the other person already has it. Many people have chlamydia. Often they don't know, because there are no signs that they have it. But it's easy for a doctor to test whether you have chlamydia. And it can easily be treated.
If you get chlamydia, it's important to get treated, otherwise you may not be able to have children. You can be tested by your GP, or by the GUM clinic in a hospital. If you live in the USA, Planned Parenthood can help you.
You can greatly reduce your chances of getting chlamydia in the first place if you use a condom each time you have penetrative sex.