Seriously?
Okay, you know those handheld electric jigsaws? The jigsaw blades that go in those have different shank types. The shank is the part that locks into the jigsaw and allows the saw blade to be held in place securely while you use it.
T-shank is a type of standardized shank shape for jigsaw blades. The other main one that you'll usually find is u-shank.
If you look at a Leatherman Surge, you will see that it has a T-shank adapter. It's the part where you can attach either the saw blade or the diamond file, depending on which one you need to use at any given time. Those two tools are both made by Leatherman and they both come with the Surge, but you can also buy replacements separately. The important thing is that since the Surge has this T-shank adapter, it means that you can also use pretty much any third party made T-shank jigsaw blade with your Surge.
T-shank jigsaw blades come in a huge variety of different types and lengths. There's ones designed only for cutting specific types of materials, or ones for a wider general purpose range of materials, ones with carbide teeth, and all of them ranging in size from 3 inches up to 10 inches long. Having a T-shank adapter on your multitool opens up the capability to use any one of those different kinds of jigsaw blades.
On top of that, it allows people to design and create their own unique tools to fit into the T-shank adapter. You'll find some examples of that on shapeways, where they have some 3d printed metal tools using a T-shank.
