If something is solidly, it is very strong and not likely to move or break easily.
Solidly is a good thing. It means something is very strong and safe.
Examples:
The big rock is solidly on the ground.
She built the wall solidly so it won't fall down.
He hit the ball solidly with the bat.
I am solidly sure that I can lift this heavy box.
The superhero is solidly strong, he can lift a car!
The house is solidly built, it won't fall in a storm.