DiracSlash[p] is the contraction (FourVector[p, ] DiracMatrix[]). Products of those can be entered in the form DiracSlash[p1, p2, ...].
See also: DiracGammaExpand, DiracGamma, DiracMatrix, DiracSimplify, DiracTrick, GS, GSD.
This is q-slash, i.e.,
The FeynCalc Book | DiracSimplify2 | DiracTrace |