When you have a networked three-way set up, one of the switches will be the master that controls The current to the lights. The other will be an auxiliary switch (also called an accessory switch, a remote switch, an add on switch – – different manufacturers use different terms). The auxiliary will not control the current to the lights itself – – it will communicate with the master and tell the master to turn them on or off.
See the device class features FAQ for discussion of light switches. The light switch part starts around post 40 (this is a clickable link)