Braxton Hicks?

What exactly are BH? And how do they feel? I've heard they feel like period cramps, but also that they feel like baby is curling up in your lower stomach. Sometimes throughout the day especially when I'm at work it feels as if my baby is like turning in my tummy with his back against the wall of my stomach and it's super uncomfortable and makes my stomach really hard and tight. It's pretty confusing to me. Btw I'm 20+3