Question

what do you mean by concave lens and convex lens

Answer 1

Convex lenses are thicker at the middle. Rays of light that pass through the lens are brought closer together (they converge).
A convex lensis a converging lens. When parallel rays of light pass through a convex lens the refracted rays converge at one point called the principal focus.
hope it's helpful

Answer 2

Convex lens refers to the lens which merges the light rays at a particular point, that travels through it.

Concave lens can be identified as the lens which disperses the light rays around, that hits the lenses