Welcome to homebrewing! You should be able to find tons of answers from more credible sources on here but the quick answer is this: when you bottle the majority of beers and use sugar to help carbonate it, you get a layer on the bottom of the bottle. This is a combination of yeast and other stuff that settles out of the beer. You can decrease this amount by filtering your beer (most home brewers don't), use clarifying agents like Irish Moss during the boil, or just leave the beer longer in the primary fermenter. When I was new to home brew, I rushed the process to much. Once the beer is "done" fermenting, leave it in there another week or more to help settle everything down, then transfer to a secondary or go straight to the bottles. If you bottle your home brew, you are almost always going to get some sediment at the bottom.
Cheers