can some one explain to me what this do?

Basically if you were to draw a line lengthways through the centre of the suspension coil then caster angle is the angle this line would meet relative to vertical (so if the spring is directly above the centre of the wheel then caster angle = 0).
Increasing caster improves straight line stability, increase the self-centring effect if you let the wheel go and increases the effect of camber when the wheels are turned (so improves grip whilst cornering but without impacting on straight-line speed). However increasing caster can lead to slightly less steering response (ie. slower turn-in).
If you look for the set-up guide in the LFS-wiki, that'll probably explain it a lot better than I have!

Not quite Si01.

Castor is the angle between vertical and the line between the two pivots of the wheel upright. The coil inclination doesn't necessarily have anything to do with it.

However, the dynamics you mention are about right. Too much castor can lead to heavy steering or excessive camber whilst turning. You can also use lots of castor and not much static camber to get proper camber control in important turns but minimal camber in a straight line for good braking stability.

Some people prefer maximum castor, others (like me) prefer a lower value.

can anyone show what it does in pic?

There is a description in the LFS manual and also a description, with pictures, at the link in my sig (Raceline Central) .

Quote from mxpxun :

can anyone show what it does in pic?

This bike has EXTREME positive caster.

Castor angle also effects dynamic camber, which is the effect of increased wheel camber as the suspension load is increased. Too much castor can lead to a loss of traction under load or extremes of cornering.

To see this in action, set the ride height on both front and back wheels to maximum, then soften the springs on both to get some decent suspension travel when the car is dropped. Set a low castor angle, drop the car and note how much camber you get on the wheel. Then, set the castor angle high and drop the car again. You should notice a change to the camber angle, although I forget which way it works. I think more angle towards the rear of the car gives more dynamic camber.

If you're going to do the above, create another setup, I don't want to be blamed for borking someones race setup.

EDIT:- Wiki Linky

Too much caster can result in more speed scrubbing off in fast long radius corners.

Castor angle is the angle at which Beavers (Castoridae) bite chips out of the tree trunks.

Quote from wsinda :

castor Angle Is The Angle At Which Beavers (castoridae) Bite Chips Out Of The Tree Trunks.

Google helps, I think that this is the most easy way to explain it.