working through some problems on complex analysis and have no idea what's going on here if someone can give an explanation i may be able to solve,

If f(z) = u(x,y) +iv(x,y), where z = x + iy,

A level curve is a curve of the form u(x,y) = c , c= constant

I don't understand this concept of the level curve any explanation would be greatly appreciated,

Cheers