what is clipping?

[CLOUDspawn]

Hello,
I was looking at the q3dm1 sample that came with GTKRadiant.
And there is all this stuff called PLAYER CLIP, BOT DO NOT ENTER, ETC.
What is that, and how does it effect the *.aas file?
,Thanks

[dONKEY]

Clip brushes are invisible barriers used to either block or smooth player movement:
Player_clip blocks players and bots.
Bot_clip blocks bots.
Weap_clip blocks everything, including weapon fire.
Clips help the bot navigation file by simplifying the playable area. As a general rule, bot_clip everything that bots cannot reach. Most especially bot clip all patches(curves), as the bspc really hates curves.
Player clip is generally used for smoothing geometry so players don't get snagged up as they run around the map, but remember player clip also affects bots.
Bot Do Not Enter is a bit of a 'fuck it I give up' move actually. Sometimes bots just act dumb and it's easier to avoid that problem area.

[CLOUDspawn]

Let's say I want to simplify the playing area and block players and bots from bumping into a wall? Let's say the wall is 4 units thick? And 50x50 big. And let's say when players are in the game they can only see one side of the wall. Do I player clip just the face with a SHIFT+CTRL left mouse button? Or do I select the wall with SHIFT left mouse button, press space to copy it and make the shader occupy the same space as the wall? Thanks

[dONKEY]

Clip brushes are separate brushes from the visible geometry. Encase all the brush or brushes you wish to simplify or smooth within clip brushes.

[dONKEY]

I cover a lot more stuff in this particular tutorial, but it explains clip brushes a bit more, and gives pictures too
http://www.leveldk.co.uk/tut7.htm