Tightening is probably the best way to put it. I also didn't know what it was until I got one when my midwife was here. It feels like your stomach muscles go all tense, almost like the baby is pushing, but it feels more symmetrical iykwim.

Mine started at 24 weeks and got stronger and longer as time went on. The day before I went into labour they were pretty full on, and I had to breathe through them...

GL with the pregnancy!