Okay, after reading the documentation of move_and_slide. It seem the veloicty is multiplied by a number delta. It says:
l"inear_velocity is the velocity vector in pixels per second. Unlike in move_and_collide(), you should not multiply it by delta — the physics engine handles applying the velocity."
To add to your answer, a linear velocity is a speed in a given direction - it's not in pixels, but in pixels per second in our case.