The phrase "multitasking button" refers to a button on a device, such as a smartphone or computer, that allows you to quickly switch between different applications or tasks that you are using at the same time. Full definition
There's also a new multitasking button that allows you to switch between running apps quickly and lots of new features. (gadgetian.com)
We've lost count of the times we've pressed the back or multitasking button without meaning to, inadvertently cancelling what we're doing or launching the recent apps screen. (pocket-lint.com)
To access it, tap the dedicated multitasking button on Nexus devices. (howtogeek.com)