It means that a poor man can only make his way in the world due to his own ability. He does not have his parents' money to help him like a rich man would.

It does not mean that it is only a poor man's wealth. It means that in the absence of wealth, if someone is able to do something very well, then there are opportunities for them to enrich themselves. So one of my ancestors was good with raising sheep about 140 years ago, despite being very disadvantaged in an aristocratic society, he became very wealthy because raising sheep was very profitable.

The sentence does not implies that --- ability is a wealth only for a poor man but it strongly suggests the fact that --- the only [possible] wealth for a poor man is ability I will add health, will, family, and what Nature offers to all of us, although this is also related to geography (location).