The magnet grade or "N rating" refers to the Maximum Energy Product of the material that the magnet is made from; and it refers to the maximum strength that the material can be magnetized to permanently hold.
Generally speaking, magnets range from N35 to N52 in grade. The higher the grade number, the incrementally stronger the magnet will be.
The grades of neodymium magnets that we normally carry are N42 and N52, a rating that is generally measured in Tesla metrics or units of Gauss Oersted (MGOe).
A magnet of grade N42 has a Maximum Energy Product of Gauss 42 MGOe, while an N52, at 52 MGOe would be incrementally stronger.