A bubble nest looks like foam (depending on how thick it is). It is usually made in one corner of the tank. If your fish isn't bubblenesting it probably just means he is too old to want to breed. It could be an indication of lethargy, stress, or sickness also but you would notice other symptoms of those in conjunction with no nesting. Some males just don't build nests unless they are about to breed.
Here are a couple examples of bubblnests:
Here are a couple examples of bubblnests:



