These are capacitors that are use most commonly to be wired inline with your speakers positive terminal to block out the bass. These are not used in high powered systems, they can only handle a low amount of wattage so be careful not to overpower these.

These come sold in pairs, enough to do one pair of speakers. You will use one per speakers positive terminal. The sound needs to go through this before it gets to the speakers positive terminal for it to filter properly. 
 
I include some extra heatshrink for you to help insulate them when installing.
 
When used on:
 
4 ohm driver the cap will give a 6db/octave high pass slope starting at 12Khz
 
8 ohm driver the cap will give a 6db/octave high pass slope starting at 6Khz
 
If you have a different driver then please refer to the capacitor calculator.