NPM stands for North Pole Marked.
This means that the side that is north has a marking, most commonly a dot or a line from a red marking pen.
If the manget is SPM, it means that the south side has a marking. Most often in the form of a dot or a line in blue.
A marking makes it easier for assembly jobs that require you to always mount the magenta in the same direction.
This could be if the magnet is to work against a sensor or if the magnet is to be mounted in a magnet system.
There are other ways to identify the north and south side of a magnet. This can be done with, for example, a pole-pen.
You can easily search for this tool or you can find it under Measure & demagnetizing equipment.
For complete information about Neodymium (aswell as the other magnet materials), see our Magnet facts page.