A solid is a thing that is not a liquid or a gas. It is hard and keeps its shape.
Solid things are usually thought of as strong and reliable.
Examples:
Ice is a solid.
The rock is solid and won't break.
The wall is solid and won't fall down.
My toy is so solid, it won't even break if I drop it!
The mountain is so solid, it won't move even if you push it!
The cake is solid and won't fall apart.
If something is solid, it is very good and does not change.
Solid can also mean very good and reliable.
Examples:
He is a solid friend.
The plan is solid and won't fail.
The house is solid and won't fall down.
The chair is solid and won't break.
The team is so solid, they never lose.
The bridge is solid and won't fall.