When something solidifies, it becomes hard or firm, like ice turning from water into ice.
When something solidifies, it is usually a good thing because it means it is becoming strong and stable.
Examples:
The hot wax solidifies as it cools down.
The pudding solidifies in the fridge.
The lava solidifies into rock.
Her plan solidifies as she thinks about it more.
The friendship solidifies as they spend more time together.
The idea solidifies in his mind as he talks about it.