Already is an adverb that can mean:
1. before now - Ex: They have already eaten.So they are not hungry now.
2. before a specified time - Ex: By the time I got there, she has already left. So we did not meet.
3. so soon - Ex : Do we have to leave already? It was so much fun!

All ready is a two-word phrase that means completely prepared. We use it to say that something, someone, or everyone in a group is completely prepared.
Ex -
1. They are all ready for the big meeting.
2. The teacher is all ready for class.