Chlorine

Reduction

What does it do?
Or, how many contaminants does it reduce? There are two rules of thumb here:
1) Look for independent testing to prove that it removes 90% or more of the chlorine in the water. NSF and Pace Labs are good sources of this information.
2) Look for a filter that has more than just one media inside. KDF effectively reduces chlorine, so most filters have KDF. Activated carbon is the second media you should look for, as it effectively reduces other organics like THMs or trihalomethanes, a byproduct of chlorine disinfection that can form nasty breathable contaminants like chloroform.

Will it impact my water pressure?

There are two things to consider here:
1) What is the flow rate? The EPA prescribes the maximum allowable flow rate for shower heads at 2.5 gallons per minute. Look for a filter that operates at that flow rate or lower.
2) Look for an upflow design. An upflow design will reduce channeling or clogging in your filter by forcing the water up through the media, against gravity, as opposed to down where media can collect or clog at the bottom of the filter. Also, upflow designs are engineered to maintain shower height.