Translation:There is a glass.
"Glaso" refers to a glass (drinking vessel); "vitro" refers to glass (the material). So to say "it's [made of] glass" one might say "estas vitro".
I think that this sentence is wrong. Unfortunately I cannot report it now as I don’t know anymore in which lesson it appeared.
One can say “Estas varma tago” translated as “It is a hot day”. And one can say “Ĝi estas glaso” translated as “It is a glass”.
While “it” in the first sentence does not really refer to a thing, the word “it” in the second sentence refers to an object. (With object I don’t mean a grammatical one.)
The differentiation is a bit murky since you may choose to omit anything part of grammar in Esperanto, even though it is not recommended since it will mess up the meaning probably.
I don't know why this was downvoted; it's a valid question.
bluematt said it first, but in short: glaso refers to the drinking vessel itself, not to its composition, which would be vitro.
Therefore, since the glass in question is a countable item, when translating this sentence into English you have to indicate the quantity, which we do here by using "a".
So why don't we translate it as "cup"? Wouldn't that clear up a lot of confusion?
So is Esperanto like Spanish in that the subject of the sentence can be omitted if it's a pronoun, or should this be "Gxi estas glaso"?
Officially, not often, although in general use it can be.
That would depend on context. e.g. If there was a box, and you asked "what's in the box?", someone might reply "estas glaso" (a glass exists [in the box]). If there was something here, and something there, one might say "tie estas glaso" (a glass exists yonder). These roughly translate, with emphasis, to "there IS a glass" and "THERE is a glass" respectively.
'There' is fairly ambiguous in English, not so much in Esperanto.