In understanding the expression ignore the "round" function – that is needed to avoid differences due to rounding errors, but doesn't affect the understanding.
We are comparing the absolute size of the raise (holdem_hand_player_detail.amt_p_raise_made) with the size of the raise as a percentage of the pot (holdem_hand_player_detail.val_p_raise_made_pct).
If the small blind has limped and everyone else has folded then the pot is 2 BBs before the big blind acts. If another player limped and the small blind folded then the pot is 2.5 BBs. If another player and a small blind both limped then the pot is 3 BBs.
We can use that information in conjunction with the absolute size of the raise in BBs and the percentage raise to determine the number of limpers.
See
this thread for calculation examples.
We need to use "round" around each part if we're going to do exact comparisons using =, since we using divisions and can end up with very slightly different decimals.