This describes how the metal conductor inside a cable is made up.
A solid conductor is a single, solid rod of metal. It is economical, holds its shape, and terminates cleanly under screw terminals — well suited to fixed building wiring run through conduit, especially in smaller sizes.
A stranded conductor is made of many thin wires twisted together to form one conductor of the same total area. Bundling thin strands makes the conductor far more flexible and better able to tolerate bending and vibration without fatigue. That is why stranded conductors are used in flexible cords, appliance leads, multicore cables and anywhere the cable must move or be routed around tight bends.
Both carry current equally well for a given cross-section; the choice is about flexibility and handling. Fixed wiring often uses solid or lightly-stranded conductors, while flexible and appliance cables use finely-stranded ones. See how to choose the right cable for the full picture.