The market level is defined by how you have leveled up the businesses in the market. Basically, the market level is the same as the lowest business level of the market. When you level up the business in such a way that all of them are on the same (or higher level), the market will reach a new level and it will receive a market-specific boost. For example, if three businesses are on level 4, one is on level 3, and the last is on level 2, the market level is 2. If you level up the last business from level 2 to level 3, the marker level rises also to 3 and the all the businesses of the market will receive boost.
You can see the market level in the map marker of the open market as well as when you open the market view, the level is shown in front of the market name.