This is the 476th most frequent German word.
schneller
"Faster"
In this instance, "schneller" is used adverbially, modifying the verb "sprechen" to indicate a quicker manner of speaking.
Kannst du etwas schneller sprechen?
Can you speak a little faster?
Here, "schneller" is used as a comparative adjective comparing the speeds of the train and the car.
Der Zug ist schneller als das Auto.
The train is faster than the car.
Here, "schneller" functions as an adjective describing the noun "Vogel" (bird), agreeing in gender and case.