The suffix -ance, when added to a verb, makes a noun that indicates the state of being related to the verb. "Accord" as a verb means to agree or reconcile. "Accordance" then is a noun meaning agreement or harmony.

Though "accord" is more often used as a verb, it can also be used as a noun. When used as a noun, it has the same meaning as "accordance". "You should dress up in accordance with the office policy" is the same as "You should dress up in accord with the office policy".