French: what does "d'accord" mean?

You would use the expression d'accord when you agree with something. 
For example:
Oui, je suis d'accord avec vous. - Yes, I agree with you.
It can also translate as sure, okay, of course, naturally, alright, etc.

Doulingo is good. you can also watch movies that are in french, i am using this tactic  for japanese  and it a lot. listing to music with  french  lyrics would work well too because  you could absorb  the words while having fun.hope this ! : )

4and 5 are the correct details