A solid, as opposed to a liquid or gas, has a size and shape to it. It doesn't flow like water or disappear into the air.
At first glance, solid appears to be a simple word, but it really offers shades of meaning. A solid friend is dependable. If you give a solid performance, it was very good, though not quite great. Likewise, if you achieve a solid win, you win by a large, but not huge, majority. In a less figurative use, if a piece of chocolate is solid — as opposed to hollow — it is chocolate throughout.