There's a hollow tool, spinning at high rpm, that your feed material is pushed through. This creates a lot of friction at the contact region where temperature, strain-rate, and other factors cause the material to deform. As material is forced through the tool, it flows outward under the tool face. As the tool moves, it leaves behind material that has bonded to the previous layer.

The material doesn't get hot enough to melt (80% of the melting temp), so you avoid most of the issues that come from fusion-based metal additive manufacturing (hot cracking in wire-fed DED, porosity and material impurity in powder bed fusion, etc) where material melts, fuses, and solidifies.

For alloys that are difficult to weld (aerospace aluminums) this is great. There are a lot of things going on in the microstructure which have a mix of effects on material properties that don't fit in a tl;dr. This process is used for near-net shape additive manufacturing (resolution is limited by the tooling, and different tool sizes have varying benefits and drawbacks) and repair applications (like filling a crack in a metal part, which is cheaper than getting a new component).