If you arrange your contigs from longest to shortest, what is the shortest contig in the smallest subset of contigs that covers at least half the expected genome length?
An example with arbitrary units...
You have a set of contigs of lengths 30, 20, 20, 10, 5, 5, 1, 1, 1, and many contigs < 1. Your expected genome length is 150. The smallest subset of contigs that covers at least half the genome length is [30, 20, 20, 10]. The smallest contig in that set has length 10, so NG50 = 10.
Note that the sum of the lengths of the contig subset is 80, not 75. But if you take away the smallest contig in that subset (10), then the sum is 70, which is less than half the genome length. So the sum of the lengths of the contig subset need not be exactly half the expected genome size, but at least half.