To solidify is to make something solid, like water becoming ice, or a plan becoming more concrete. If you’re meeting a friend, you should solidify the plan before you go and decide where and what time to meet.
To solidify is to become a solid, like lemonade solidifying into a popsicle, or dissolved sugar solidifying into yummy rock candy. Also, this word can apply to non-physical things. If a couple gets engaged, they're solidifying their relationship. Getting a promotion could solidify your career. Anything becoming firmer or more real is a type of solidifying. In any sense, whatever gets solidified becomes stronger.