Definition of mismatched
Mismatched means that two cards are not the same suit. By suit, we mean the four suits of the four families of the classic 52-card deck (hearts, spades, clubs and diamonds). If you receive an ace and a king in spades, your cards are said to match. If you receive an ace in spades and a king in clubs, your cards are said to be mismatched. Any combination that is not spades, hearts, diamonds or clubs is considered mismatched.
Mismatched cards vs. matched cards
If you've ever played poker, you know that as a rule, people like matching cards. While you shouldn't overvalue them either, it is true that matching cards will give you an advantage over non-matching cards. For example, let's take a hand like 8-9 mismatched and compare it to a hand like 8-9 suited. We will calculate the equity of each hand against a random hand. 8-9 mismatched will have a 48% equity against a random hand while a hand like 8-9 matched will have a 51% equity. In the long run, this is the difference between losing a little and winning.