What do contractions feel like?

Hey, I’m sorry if this is a weird question but I’m generally curious. My stomach has had this tight feeling to it all day and it’s just super uncomfortable, nothing unbearable though. I’m currently 25 weeks pregnant and my OB always ask me if I feel tightening in my stomach. Should I be concerned? Also wondering what they felt like to you mommas, so I can be prepared. This is my first pregnancy and I can’t went in blind.