@alexan I know this is old, but it's the first thing to appear on google.. so I figured I should reply here.
This is the description of tab slash as I was able to find with some googling around.
What is tab slash Opera?
For the tab hoarders among us, Opera GX has a new useful feature called Hot Tabs Killer which lets you identify the most resource-draining tabs and slash them with a sword to make sure the browser doesn’t overburden your PC or laptop.