Clear means clean and uncluttered, without distractions or confusion. If your explanation of relativity is clear, it's easy to understand. If the sky’s clear, there isn't a cloud to be seen.
As a verb, clear implies getting rid of obstacles. You can clear your head (get rid of anxiety and upsetting thoughts); clear the way (remove obstacles in your path); or clear the trees to make a clearing where you can pitch your tent. If you own your car "free and clear," it's all yours — you don't owe anything on it. When you arrive in a country, you clear customs when officials check your papers and belongings and send you on your way.