Use the adjective meaningful to describe something that has a purpose, like a political rally for a cause you believe in strongly.
You would probably say that your backpacking trip across Asia was meaningful, or worthwhile, but you might not describe three hours watching bad TV shows as meaningful. When something is important or significant, it is meaningful, or "full of meaning." Meaning itself comes from the word mean, which has its roots in the Old English mænan, "to signify, tell, or complain."
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