It's a challenge.

Meaning

It's a viral activity people try and share online.

Examples

• John: Have you tried that ice bucket challenge yet?
• Alice: No, but I've seen it all over. <b>It's a challenge</b> that's very popular.
• John: It looks fun, but also kind of silly.
• Alice: That's the point! That's why <b>it's a challenge</b>. Everyone's doing it.