A beam is a horizontal member and it takes load by bending itself only. So you know very well the beam will either sag(like a valley) or hog(summit forms ). In both the cases upper fibres of beam undergoes compression(in sagging) or tension(in hogging) and lower fibres goes the same in reverse order.
As we know that concrete is very good in compression but is a brittle material hence weak in tension. So to overcome this weakness steel is provided in it. Steel provides the beam flexibility it requires for bending.
Sagging is also called positive bending and hogging is called negative bending.
If sagging occurs- it means beam is undergoing positive bending moment so the reinforcement provided for this moment resistance will be termed as Positive Steel.
If hogging occurs- it means beam is undergoing negative bending moment so the reinforcement provided for this moment resistance will be termed as Negative Steel and this steel is always provided in tension zone. or part undergoing hogging moment.
There may be a case in which beam is undergoing both positive and negative moment, in that case there exist a point of contraflexure, to negate the effect of this point Negative Steel is provided. Like practically almost every concrete joint is either fixed or continous so definitely there will be a contraflexure point near the supports. Hence the type of reinforcement provided is termed as Negative Steel.