This is how about sounds when people talk fast and drop the first little sound. You will hear it in songs and in very easy, friendly talk: How 'bout we eat? or It's 'bout time. It does the same jobs as about: it can mean “more or less” ('bout ten people) or “on the subject of” (talkin' 'bout you). Though actually, it is not a different word with a different meaning — it is only about in working clothes. You write the hole with an apostrophe so readers can see a sound has gone. Keep it out of school essays and careful letters; there you still want about.