When you set up a wired network using Ethernet cables (e.g. Cat5e or Cat6), there are 2 types: solid and stranded. The rule of thumb is that you use solid cables when they’re not going to move, e.g. between a wall socket and a patch panel. You use stranded cables when they will move, e.g. between a desktop PC and a wall socket. Solid cables are better over long distances, while stranded cables are a bit more flexible and they’ll probably survive being run over by an office chair.
However, what’s the actual difference between those types? In a way, they’re both solid, in the sense that they’re not liquid or gas. They also both contain thinner wires inside (twisted pairs). The difference applies to the copper wire once you remove all the plastic sheaths.