Zero beating is just ensuring that you are operating on the same frequency as the station you are receiving. If you don't do this, the other station may not hear you when you reply. For example, if the frequencies don't match and he has his filters set narrow your signal may get filtered out.
This is one of the things I like most about the FT-897D. It is extremely easy to zero beat another station. There is an LED on the front panel marked 'CW-T'. Just tune the VFO knob until the LED blinks in time with the received signal. That is it. You are zero beat. Works phenomenally well.
As mentioned by AA4PB, with most rigs if you are zero beat the frequency of the received CW tone matches the frequency of the sidetone when you transmit. So, you should be able to listen to their signal and then key your transmitter and the two tones should be the same. With some rigs, that is how you zero beat. Believe me, that little flashing LED is a much easier and much more convenient way to to this. I really don't understand why all manufacturers don't adopt this practice.
In fact, I liked the feature so much, I even found a way to add it to a QRP rig I have.http://www.wb3aal.com/Pages/K6XX/K6XXCWIndicatorKit.htm