'inroad' in this context means an incursion, penetration, etc., and market inroads means penetration (often limited or initial) into a market, or in other words acquiring a relatively small part of a market, usually at the expense of other players or products in the market

sample ref:

The study concluded that gasoline- and diesel-fueled internal combustion engines will continue to dominate the vehicle market until that time. Fuel cells may start to make some market inroads near 2020, but only if development efforts now under way meet success.