'Ramp Mode' is the name used for the state the game puts your character in when you interact with a ramp. This state has some interesting characteristics, such as increased vertical velocity when trick jumping.
The reason why this is useful (outside of the ramp itself) is that some characteristics of Ramp Mode do not go away until you land on the ground. If you jump off of a ramp (touching the ramp puts you in Ramp Mode) and then grind a rail, you will still be in Ramp Mode.
The most useful application of this is that jumping off of a downwards rail will give you the increased vertical velocity of jumping off a ramp, allowing you to jump much higher off of a rail than normal.