Yeast; they drop out of suspension once they're done eating the priming sugar.
You're stuck with it, unless you're set up to force-carbonate with a CO2 tank, but after 48 hours in the fridge, it usually sticks to the bottom of the bottle pretty well, so, you can avoid pouring it into your glass without leaving too much beer in the bottles.
Of course, if you don't mind cloudy beer and a slight flavor of yeast, you can just pour it into your glass and drink it, it's not bad for you in any way, it's just not typical to have suspended yeast in most styles of beer.