Here is the official explanation of what the Activity is, which can be found on the same page where submissions should be uploaded:
This is part of a series on LEGO Ideas that gives little tasks for builders to get involved with – there are no prizes, they are just a way to get involved with building something quick for fun. Each challenge is intended to be something quick and fun, unlike the more in-depth building required for a competition or product idea.
The deadline for this Activity is September 23 at 10.00am CEST.
The last LEGO Activity asked fans to build something sporting.