If I'm buying Eibach for example .. what does 500lb & 300lb mean? Why did Kev get 500 & 375?

 

A spring's "rate" is the amount of weight required to compess the spring. Thus a 500lb spring means 500lb/inch meaning it takes 500 lbs to compress it an inch. Stiffer springs have higher spring rates, softer ones have lower spring rates.