From the wiki regarding Conquerer's Periapts:
◾Boosts Strength by approx. 20% for 60 seconds.
◾Can be stacked up to four times (multiplicative).
And for convenience, some has done the math:
"4x Conqueror's Periapt boost your strength by 2.0736 times
AKA, if your strength is 3,000 it would be 6,221 after taking 4 of these
If it was 5,000 it would be 10,368"
This is the same for Demon's Periapts as well (which raise Magick).
It takes the total number of strength (or magick) points one has (Base + Accessory/armor boosts + Weapon + augments), then attaches the multiplier. And in the easiest terms....more means high strength or magick, and kills things faster.