You can read more about how Activity scores work here
To update the threshold, head to Settings in the left hand menu:
Then choose "Threshold".
Edit the threshold point to something that will be manageable for you and your team. For example, if when you set a score of "20", you have 150 ideas past that point already, it may be too low. You may wish to set the bar higher to only get the most interacted-with ideas to the surface!