Learn Turkish: Su ve ekmek al. - Buy water and bread.

al is the bare root of the verb almak (to buy / to take). In Turkish, to give an informal command to one person, you simply drop the -mak or -mek from the dictionary form. The bare root that is left behind is the command!

Does al mean "buy" or "take"?

It means both! The verb almak is used for "to buy", "to take", and "to get". You rely on context to know which one makes sense. If you are heading to the store, it means "buy". If someone is offering you food from a table, it means "take".

Why don't su and ekmek have any endings? Shouldn't they take the accusative case as direct objects?

They don't have endings here because they are indefinite. The sentence means "buy water and bread," rather than "buy the water and the bread." In Turkish, direct objects only get the accusative case ending when they are specific and definite.

Why is the action (al) at the end of the sentence?

Turkish naturally follows a Subject-Object-Verb word order. Even in a simple command where the subject is implied, the object (su ve ekmek) comes first, and the verb (al) waits until the very end.