" 20 m/s² " means that if gravity is the ONLY force on the object
(the object is in 'free fall'), then its speed increases by 20 m/s
every second.
That's the answer to your question. Now, let me ask you
another one:
How does a speedometer tied to a falling rock work ?
How can it measure the rock's speed ?
Maybe one way would be to have a little tiny propeller on
the front of the speedometer, and it could measure how fast
the propeller is spinning as the rock falls through the air ?
Great idea. But we already said the rock is in free-fall,
so there's no air resistance, we can't have any air, and
there's nothing to spin the propeller.
How would you do it ? How can you measure the rock's speed ?