Solar panels are devices that convert light into electricity. They are called "solar" panels because, most of the time, the most powerful source of light available is the Sun (called Sol by astronomers). Some scientists call them photovoltaics which means, basically, "light-electricity."
A solar panel is a collection of solar cells. Lots of small solar cells spread over a large area can work together to provide enough power to be useful. The more light that hits a cell, the more electricity it produces, so spacecraft are usually designed with solar panels that can always be pointed at the Sun even as the rest of the body of the spacecraft moves around, much as a tank turret can be aimed independently of where the tank is going.