Learn Spanish (Spain): Chicos, entrad y descansad.

Because this sentence is giving a command to a group of people informally (vosotros). In Spain, when you tell a group of people to do something, you take the dictionary form of the verb (like entrar for "to enter" or descansar for "to rest"), drop the final -r, and add a -d.

What would this sentence look like if I were only talking to one boy?

You would say Chico, entra y descansa. To give a command to just one person you know well (tú), you use the regular he/she/it form of the verb in the present tense.

How is the "d" at the end of these verbs pronounced in Spain?

It is pronounced very softly. It sounds similar to a soft English "th" (like in "father" or "this"), rather than a hard "d" like in "dog".

Does chicos always mean "boys"?

While it literally means "boys", it is very often used to mean "guys" or "kids" when addressing a group, even if the group includes both males and females.