This filter applies a strict filter on the outputs from the Influencer and Network Analysis widgets when various People or NPI filters are active on the dashboard.
Here's an example of why this is desirable:
Imagine you want to see top influencers from California only, so you add the Location filter and select California. But the output in the Influencers or Network Analysis widgets still includes people outside California, how did that happen? The reason for this is that the Location filter limits the analysis only to tweets from people in California, however, people in California may still be influenced by profiles outside California (for example, top influential cardiologist among people in California may be Dr. Deepak Bhatt from Harvard).
When applying the "strict filtering", you make sure the output only includes profiles that match the active People or NPI filters on the dashboard.
Dashboard filters where strict filtering is available: