This is a phrase used in baseball when a pitcher takes action, or adjusts their performance against the hitter in order to ensure they aren't gaining what may be seen as an unfair advantage over the pitcher. It is also used to describe a pitcher throwing the ball aggressively close to the batter. In the former usage, throwing the ball inside on a batter is one way to keep them honest. However, the main imperative behind keeping a hitter honest to keep them guessing as to what pitch is going to be thrown and where. A pitcher combats this by mixing up the speed, timing, and order of their pitches.
It is important for pitchers to mix up their pitches because batters are quick to jump on consistencies in a pitcher's pitching habits in order to get a jump on the ball. If a batter knows a good pitch is coming, they can be ready to swing at precisely the right time. In baseball, signals are used to communicate pitch type between the manager/catcher/pitcher. There are methods teams use to disguise signals from the other team, but opponents are always looking to decipher them for a competitive edge. Mixing up pitches is especially important in the modern age because scouting reports and digital pitch tracking technology is making it easier to analyze pitchers.