The Launch angle describes the initial trajectory of a batted ball as it leaves the bat. In association with the ball's exit velocity, it can provide an assessment of how far a ball is liable to travel - and hence its probability of being a home run.
It is a well-established principle of physics that the distance which an object benefiting only from initial propulsion will travel depends on the angle at which it leaves. The more the angle approaches the vertical, the more the object will simply reach a very high altitude and then fall back to the ground without having traveled far from its launching point. In baseball terms, this would be a pop-up. Conversely, if the object is hit at an horizontal angle, it will tend to fall towards the ground and hit it while still travelling at a high velocity. That is a line drive. Thus, when what is sought is distance, for example when one is looking to hit a home run, an angle just below 30 degrees is optimal, as it will maximize the distance traveled relative to the force of the initial impact.
While the underlying principle has been known for some time and has been part of the education of artillery officers for decades, there was no capacity to measure the launch angle in baseball until the introduction of advanced tracking systems such as Statcast in the 2010s. The data derived from this led to a better understanding of the types of swing that produce great distance, and given the importance placed on hitting homers by most analytics systems, it led to conscious efforts to modify batters' swings in order to optimize the launch angle. Batters who claimed to have increased their home run-hitting prowess by deliberately attempting to modify the launch angle on balls with which they make contact include Yonder Alonso.