You say that something "is on the market" when it's being sold publicly.
Examples of things that can be "on the market" include:
- houses that are being sold
- products that a company is selling
- a company that is ready to be bought by another company can be said to be "on the market"
It's a bit of a joke, but you can also say that a person is "on the market" if they've just become single and are ready to date people:
Watch out, ladies! I'm back on the market.
When you first start to sell an item, you "put it on the market".