Indefinite articles in German refer to unspecified persons, objects, ideas etc. There are three indefinite articles: "ein" (used for masculine nouns), "eine" (used for feminine nouns) and "ein" (used for neuter nouns). Their equivalent in English is "a" or "an".
Just as in English, the indefinite article disappears in the plural.
It＇s "ein Glas" (singular) but "Gläser" (plural)

See the examples below:

first column:
masculine nouns

second column:
feminine nouns

third column:
neuter nouns