"Hit or miss" is an English idiom that refers to something that is effective sometimes but ineffective on other occasions. Unlike things that are reliable in the results that they produce, things described in this manner produce results that are nearly impossible to predict. In sports, the phrase refers to a player who comes through occasionally but fails just as often. As a matter of fact, the phrase "hit or miss" comes from the sport of baseball, referring to batters who either make contact with the ball or swing and miss.
There are certain times when someone speaking English may find it favorable to use phrases that gain their meaning through familiarity in the culture. These phrases are called idioms, and the meanings they have may not be easily guessed by those unfamiliar with them. This is because those meanings might stray somewhat from the literal definitions of the words used. Many of these idioms in the last century or so come from the world of sports, which has such a powerful hold on popular culture. One of those sports-related idioms is the expression "hit or miss."
If something or someone is described in this manner, it means that they are essentially unpredictable. It is very possible for them to succeed when they set out to do something. On the next occasion, they are just as likely to fail. Since the phrase comes from sports origins, athletes are often described with this idiomatic expression. For example, someone might say, "I can't begin to predict if he'll make this shot, because he is so hit or miss."
This phrase is also malleable enough to describe just about anything that produces some sort of outcome. Anything that produces just as many positive results as negative results might be described in this manner. Since there is a balance between good and bad implied by this phrase, it can also be used to describe something that is mediocre or average. As an example, imagine the sentence, "We can go to that restaurant if you want, but I'm warning you that the food is hit or miss."
In the sport of baseball, a batter tries to hit a ball thrown by the pitcher. On some occasions, the batter might hit the ball. Other times, he might swing and miss. This characteristic of the sport is where the phrase "hit or miss" originated and is also where it gets its accepted meaning as an idiom.