A traveler wire is used between two switches that control the same lamp. It’s easy to get around that by using a “virtual three-way“ (“virtual two-way” if you are in the EU). But I suspect that’s not what you’re really asking about.
If you are in the EU, a “two wire“ versus “three wire“ light switch is one that does not have a neutral.. A neutral is required for most smart switches to power the radio. But it is not required for all.
If you’re in the EU, see the following recent thread:
FAQ: Light switch no neutral (UK) (2019)
If you’re in the US or Canada see the following:
FAQ: SmartThings-Compatible Switches that Don’t Require Neutral (US, 2019)