The miter limit is the limit when a join switches from mitered (pointed) to beveled (squared-off).
You can calculate the miter limit by dividing the miter length by the stroke weight. If the length of the miter equals a given number of times the stroke weight, the join will be beveled.
Suppose the stroke weight of a line is 2 pt. and the miter limit is 4. As soon as the length of the miter is 8 pt., the tip of the angle will be squared off to a bevel join. Logically, a miter limit of 1 will always be a bevel join because the miter length is always more than the stroke weight.
To allow “pointier” angles, you increase the miter limit. Obviously, the miter limit does not apply to joins of which the style has already been set to round or bevel.