Depending on the context it can mean: What subject is that?
Welches ist das schlimmste Fach = Which is the worst subject
Depending on its function in a sentence is can be translated with das, welches or dass.
English: "the subject" can have many different meanings. So we find in German: "das Arbeitsstück", "das Aussetzen", "der Betreff", "das Fach", "das Fachgebiet", "der Gegenstand", "der Gesprächsgegenstand", "das Gesprächsthema", "das Lehrfach", "der Leichnam", "das Medium", "der Proband", "die Testperson, "der Satzgegenstand".
Hans Kotte has written: 'Welches Papier ist das?' -- subject(s): Paper, Dictionaries, Sample books, Polyglot, German language
(das) Pferd = (the) horse
Das Ende is German for the end.
Das Mutterschiff' = the mother ship
Was ist das = what ist that
"That is not" is "das ist nicht" in English.
Das Geschenk
(das) Tagebuch
It means "the" ^this is kinda right^ Das depending on its context can mean, this, that, or the. Like many German words 'das' encompasses more of an idea than a single meaning. Das basically means a singular thing.