The older grammars used the term “mode”. The use of different mood verbs allows a speaker to express their attitude toward what they are saying (for example, whether it is intended as a statement of fact, of desire, of command, etc.). Advanced students, see Wallace’s detailed definition GGBB p. 442ff.
Most verbs are in the indicative mood which is the mood of simple declaration or question:
Did you sleep last night?
I slept well last night.
The subjunctive mood implies possibility or potential. In English, use a helping verb like “might” or “would”.
If you go sailing, you must wear your life jacket.
I might go fishing tomorrow.
In Greek, there are four moods:
Mood is not used all that much in Hebrew but see here.