"Over the years we've expanded Gmail's communication abilities by allowing you to make voice and video calls to other computers and more recently call phones" Richard Dunn software engineer at Google wrote.
"Until now though you were limited to making one call at a time. Today we're excited to relax that restriction and allow you to make or receive multiple calls in Gmail" he announced.
Obviously real phones have this possibility as well. But even those are more limited than what Gmail is promising now Gmail's only restriction to the number of concomitant conversations is a maximum of two outgoing calls to actual phones.
The feature works in a way you should be familiar with. If you're already engaged in a call and get a second one you can pause the first and answer the incoming call.
Another benefit of this new feature is that you can place any call on hold at any time so if you need to take a small break you don't need to end the call you can simply place it on hold and return to it later.
"If you’re in a call and make a second one your first call will be put on hold while you talk on your new call. You can switch between calls by pressing the Resume button on the call you want to talk on which will automatically put the previous call on hold" Google explained.