Filtering can use a Blacklisting or Whitelisting technique to prevent some traffic going through (Blacklisting) or attempt to only allow some traffic to go through or run (whitelisting as in Application Whitelisting). 

Think of Blacklisting like straining pasta. You allow the water to run through the strainer into the sink, but the pasta remains in the strainer and unable to reach the sink. The pasta is Blacklisted.

Whitelisting is like a guest list at a nightclub. The line of people may be long and checked over more thoroughly, however the guest list can come through the red cord, through the red carpet and into the club. These guests are Whitelisted.  

Adapted from Ernst (2013)