If something is average, it's ordinary and not very special. You might say a movie was average if you thought it was just okay.
Average can describe what's true most of the time, as in, “The average rainfall in Seattle is fifty-two inches.” It can also be used in a mathematical and more precise way. On his English tests, Dylan received grades of 92%, 85% and 94%. His average grade was 90%, or the sum of the test scores divided by the number of tests taken.