Question About Condoms

This is probably a stupid question, but I've been wondering about it for a while. I know that while using condoms alone, there's about a 2% chance of becoming pregnant. That's if you use condoms correctly.
​I guess I'm just a little confused as to what they mean by correctly, and how you could even get pregnant if you used it correctly. My main question is: Is it even possible to get pregnant if the condom doesn't break? I'm assuming it's not possible, but I've always been a little curious about it. 
​Thank you :)