Activity scores are a measure of interaction with a particular idea on Solverboard. It's calculated from the number of views, follows & comments an idea gets.
Your Platform Owner can set a threshold at which an idea is deemed popular enough to assess. If an idea gets a lot of engagement from your users, then it's probably worth investigating further!
Once it reaches the threshold set, an idea's activity bar will go green, and it will be flagged to your Innovation team to suggest they review it.
You can also view an idea's activity score from the Ideaboard by hovering over a particular idea.