Common Mode Rejection (CMRR)
This choke is designed to provide the best CMRR over the entire HF spectrum, from 1.8MHz to 30 MHz.
The purpose of a Common Mode Choke is to reduce, or eliminate, common mode current flowing on the shield of coax feedline. It does this by presenting a high impedance to the common mode current, which "encourages" that current to flow into the antenna instead of back along the coax shield. How well it does this is measured as the Common Mode Rejection Ratio (CMRR).
For example, this choke has about 48 dB rejection on 40 meters. This means that only 0.0016 % of the common mode current will make it through the choke onto the feedline shield. Which is outstanding.
Common mode current flows on the coax shield because the shield is connected directly to one "leg" of the antenna. Placing a Common Mode Choke at the feed point "breaks" that connection and stops the common mode current from flowing on the coax shield. The choke isolates the feedline from the antenna.
Connecting the shield directly to one "leg" of the antenna also makes the shield part of the antenna. This affects the impedance, the efficiency, and the pattern of the antenna. A Common Mode Choke placed at the feed point preserves that antenna's "native" characteristics.
In addition, some of your transmitted signal is picked up by the coax shield, inducing common mode current on the feedline between the feed point choke and the rig. This current will make its way to the rig and can also cause RFI. Placing another Common Mode Choke near the rig, perhaps just before the coax enters the radio shack, will block this common mode current and further reduce RFI and received noise.
RFI and Noise
Common mode current on the coax shield is undesirable because it causes RFI - your signal interfering with other electronic devices.
It is also undesirable because local noise picked up by the coax shield becomes common mode current flowing on the shield which, without a choke, is conducted directly onto the antenna. This raises the noise level during reception.
This noise will also be conducted directly to the rig, and some portion of it will show up as received noise. The cure is to place a second Common Mode Choke in the feedline near the rig.
Common Mode Chokes stop RFI and reduce received noise - sometimes quite dramatically.