What does cap means? Thank you

The maximum limit you can get with an affix.

For example, the crit dmg cap is 350% which means you can put as many crit dmg affixes (crystal or epic affixes or natures) and after it surpasses the 350% number, it no longer adds crit dmg . All sets cap is 4 and no more. At that cap, you can increase all the sets by 4 so they have (5) next to them. Many affixes have caps for good reason to balance them out and make us more skillfull at deciding affixes as well as not overwhelm us.

There are affixes without caps though that are really powerful such as Elemncrit (Blight, Blistering,Frostbiting,High Voltage and Celestial) because they are only limited to your 6 gear so the max you can possibly get is either 600% or 1200% if you have 6 eternal gears. You don’t really need 6 but the amount you get without eternals is up to 600%.