Divisions - set the number of subdivisions. e.g. a value of 5 will subdivide an edge into 6 edges (creating 5 new vertices along the edge).
Max. Length - If an edge's length is less then this value it will not be subdivided. e.g. if you have a 100x100 rectangle, and Divisions set to 2 with Max Length set to 51 you will only get one subdivision as the resulting edges after the first subdivision are 50 (< 51).