What does it mean for a switch to "filter" a frame?

Filtering is when the switch already has the destination MAC in its table and forwards the frame out only that single port, instead of flooding it out every port.

Frame filtering:

• A switch populates its MAC address table by reading the source MAC of every frame it receives
• Once the destination MAC of a frame is in the table, the switch forwards the frame out that one specific port only
• This is "filtering" — the frame is kept off all the other ports

Why it matters: Filtering is what makes a switch better than a hub: a hub repeats every frame to all ports, whereas a switch sends a known unicast only where it needs to go, reducing unnecessary traffic and improving security and performance.

Go deeper:

• Host to Host through a Switch — Practical Networking — filtering a known unicast out one port versus flooding an unknown one, built up animation by animation.

• Network switch (Wikipedia) — how the MAC-to-port table lets a switch forward to just the destination port instead of repeating like a hub.