The first time it happened to me I was a bit freaked out!! It is like your lil one is having spasms or something :) Imagine yourself having the hiccups - and then imagine it on a smaller scale. It is little bumps every so often just like when you get the hiccups. My 1st child had them ALL the time, but my 2nd and 3rd didn't get them very often at all. Just one of those things! It depends on the kiddo :)
For me it reminds me of a muscle twitch only it's inside your tummy. Sometimes my baby gets hiccups 4-5 times within a few hours, so it gets annoying. Otherwise it's pretty neat. My 3-year-old says "the baby is jumping" and thinks it's funny to lay her head on my belly to feel it. Then she says "the baby jumps like on the bed, but she's jumping on my face!"