It's a piece of glassware used for handling liquids, usually in a laboratory,
although I use one as a coffee cup.
They have markings on the side, like a kitchen measuring cup, so you can
measure different amounts of liquid, and they have a lip for pouring.
They're also usually made of a special type of glass (borosilicate) so that
they won't crack when very hot liquids are poured in.
Here's a picture of a beaker with 200 milliliters of some blue stuff in it.