Here is the description from the official announcement:
Activities are small, informal warm-up challenges that we will set forth on a more regular basis than contests and are of a shorter participation period too. It’s about unleashing your creativity in a shorter burst and it’s for members who may not want to spend a whole load of time building big creations, but instead want to take an evening of their week to tinker with something new – just for the fun of it.
The LEGO Group may use some of the builds created on social media.
Initially, Activities will only be available as a Beta (test) opportunity, so just a selected few users will have access to it. The first challenge, which eligible users will be contacted about, will be “Build your finest Micro Modular Building”. Those participating have until May 13 to get involved.