Learn Spanish (Latin America): Los doctores descansan en el cuarto ahora.

Because the subject is Los doctores (The doctors), which is a plural "they" subject. For regular -ar verbs in the present tense, you drop the -ar and add -an to match ellos or ellas (they).

Can descansan also mean "are resting"?

Yes! In Spanish, the simple present tense covers both general routines ("they rest") and actions happening right now ("they are resting"). You do not need a separate word for "are" here.

Why is the plural of doctor spelled doctores?

In Spanish, when a word ends in a consonant (like the r in doctor), you make it plural by adding -es. If a word ended in a vowel, you would just add an s.

Wait, doesn't cuarto mean "four"?

Close! Cuatro means "four". Cuarto means "room" (and can also mean "fourth" or "quarter"). Mixing up the r and t in these two words is a very common beginner mistake, so keep an eye on the spelling!