Interior Players refers to the 5 guys that are covered by someone else on the line of scrimmage outside of them. For instance, in this formation:
LT --
LG --
C --
RG --
RT --
TE ---------- WR ------------
'''''''''''''''''''''''''''''''''''''''''''''''''' '''''''''''''WR''''''''''''''''''''' WR
Let the "-------" represent the line of scrimmage (I put the '''''''''' there because I can't put spaces and make the formation look correct). I
bolded the players that are considered
interior players. The reason these 5 are interior players is because they all have players on the line of scrimmage to the left and right of them. The LT does not have anyone on the line of scrimmage to the left of him, therefore he is not an interior player and is eligible for a pass. The WR on the right side does not have anyone on the line of scrimmage to the right of him, therefore he is not an interior player and is eligible for a pass.
I hope that clears it up.