Hola Celia
They mean different things, the same way they do in English.
Estas is more specific, "These chairs" =Estas sillas
Las is more general, "The chairs" = Las sillas
If you say "estas" you are referring to some specific chairs and you are probably pointing at them in front of you. If you say "las" you are either referring to chairs in general, or your listener knows which chairs you are referring to without being specific as they are the only chairs in the room, for example, so you would use "las".
Hope this clarifies it.
Inma
