NXX (where XX ranges from 01 to 99) is equal to the maximum positive integer $L$ such that all the contigs having length at least $L$, when taken together, cover at least XX% of the total length of the genome.

As a general rule, the larger the value of a given N statistic, the higher the quality of the assembly produced, since it means that we can cover this percentage of the genome using only long contigs.

One commonly used N statistic is N50.