1. It means Apple takes a good idea, like an MP3 player, which a lot of people want to buy (huge markets).
2. This means that Apple sells the idea really well.
Look at the iPod as an example. Apple did not invent MP3 players. There were others before the iPod. However, Apple saw that many people wanted to buy MP3 players, so they created the iPod. Compared to other MP3 players, the iPod was easy-to-use and marketed really well, so it "cornered the market" which means that everyone wanted an iPod instead of buying other MP3 players.
So Apple takes good ideas and sells them better than other companies.