The map has different kind of markers on top of markets (regions).
The markets where you have businesses running have violet marker on top of the region. The marker shows the level of market, its earn rate, and if there are businesses ready for level up (customer icon) or upgrade (cash icon).
If you have a market expansion (as a reward from player level up) available, the markets with a violet speech bubble and open lock shows where you can expand your business empire. If you cannot expand to a new market, the speech bubble is grey and shows closed lock. In both cases, you can tap the marker to view what businesses it contains.
The markets that are not adjacent to any open market are shown with semi-transparent lock symbol.