To concur is to agree or approve of something. If someone says something you agree with, you can say "I concur!"
Like many words with con, concur has to do with agreement and being together. When you concur, you agree with someone about something or let them know you approve. "I concur" is a formal (and sometimes humorous) way of saying "I agree!" or "I hear that!" Also, two events that happen at the same time can be said to concur. People are happy when good things concur, like when a birthday and nice weather happen at the same time.