Idk how that happened, screenshot is blow. (These are all texture under the keyword "tf", earlier they were there, but when i searched for "Door" once, they didnt show up, restarting hammer and even reinstalling tf2 didnt help.)
"tf" is a keyword. "Filter" is where you search. Think of it like filtering the names of materials by whatever you put there, such as "door." Adding the "tf" keyword mostly just hides HL2 materials, though some TF2 textures don't have the keyword.