getcontext("2d")

Question

What does it mean? (especially 2d)

Answer 1

It means you are getting a 2-dimensional context of the canvas so everything you draw is 2d!
getContext("webGl")
youre getting a 3d context if the canvas so your drawings will be 3-dimensional

Answer 2

it's used to declare the context for use in drawing the canvas object

Answer 3

It means our context is 2-dimensional or planar context