What are contractions exactly?

I thinkkk I’ve been having Braxton Hicks since yesterday, stomach gets pretty hard through out the day not painful at all, today lost my mucus plug, still thinkkk I’m having Braxton Hicks except with some pressure near my pelvic area? Will I know when I’m having contractions? Also been having low back pain for about 3 days now Im a FTM so I’m not really sure what they feel like