This is a question asked many times by customers looking to buy their first synthesiser.
Monotimbral means only one sound can be played at a time, whereas multitimbral means more than one voice at once.
Monophonic means a single note or key can be played at once. Polyphonic means more than one at a time.
For example MiniNova is Monotimbral so one sound at a time, but it can play eight notes at once (8 note polyphonic) with the chosen sound.