Literally: mind – spirit – throw – join
Alternately: Getting along with someone really well. Hitting it off. Finding a kindred spirit. Mutual understanding. Perfect concordance.
Note: For some reason it’s easy for me to start thinking that this phrase mostly applies to when two people have just met and develop a rapport right off the bat, but that’s not the case: it can also be used for people who have developed such a rapport over time.