My experience with these have been somewhat weird.
I've felt them for a while but never really known what it was. There's no surprise then that curiosity got the better of me and I googled away (I like to think of it as consulting my online midwife lol) and plenty of sites confirmed the sensation as Braxton Hicks.
Braxton Hicks, if you don't know, is where your bump goes really hard and tense, generally only for a few minutes before softening again.
Sometimes I feel a build up of pressure, but for me it isn't painful at all, although depending on the position of the baby it can be rather uncomfortable.
When this happens the tummy muscles are contracting around the baby, practicing for labour.
In fact I am having them whilst writing this. I can only hope though that I find actual labour this interesting and funny 🙈