Specular is just a fancy way of saying shiny. It means a surface is not rough, but smooth and reflective like a mirror.
A higher level description is that the light that hits a specular surface doesn’t reflect in a bunch of directions; light bounces off a surface (reflected ray) at the same angle as it hits the surface (incident angle). Of course, perfectly specular surfaces don’t typically exist in reality . . . but they’re often fine approximations in simulations (depending on application).