Photo of check rail
Picture source: Self

It's called a 'Check Rail'. Usually you can see these on the inner side of curves. If a train derails on a curve, it will tend to move over the outer rail ( The wheel will climb over the outer edge of the curve). In that case, the inner wheel will touch this check rail and stop further outward movement. You can see these kinds of rails at points and switches also, where trains switch tracks.

i think technical term used is ""gaurd rail",and yes it prevents wheels from derailing(extra stability),usually we can see them on bridges,under FOB's

Both terms are interchanged frequently. But normally, the term 'Guard rails' is used for the cases where there are 2 inner rails. These are usually seen on bridges and belo FOB's etc. The 'term 'check rail' is usually used for the cases like in the above pic, where the extra rail is on only one side of the tracks. However, there is no fixed definition for these terms I guess and I have seen people use both terms to refer to such track layouts.