Chauvinism is when someone believes that their nation, group, or ideas are better than all others. A chauvinist is a person who thinks this way and tries to prove that everyone else is less worthy or less significant. Chauvinism can be compared to a big ego, but not personal, rather collective, which blinds and prevents seeing others on an equal level.
Imagine there is a football team whose fans are convinced that they are the best in the world, and all others are just worthless. They refuse to acknowledge that other teams can also be strong. This very much resembles chauvinism in simple terms. In this case, such fans can be similar to chauvinists because their confidence in their superiority crosses the line of common sense.
Thus, the definition of chauvinism can be reduced to excessive and blind love for one's group and underestimating others. This can be harmful because it prevents understanding that all people have their values and can teach something.