Renewable energy systems generate DC power, but normal household appliances operate on AC power. The inverter's job is to take that incoming DC power, be it from solar panels, batteries, or wind turbines, and convert it to AC so you can use it.
There are many different types of inverters; the most important factor in choosing yours is your application. Will you be tied into the utility grid, completely independent of it, or a combination of the two? Is there partial shading where your solar panels will go?