If you want to get started with stenography and Plover, all you really need is an N-key rollover mechanical keyboard. But eventually, you want to think about getting an actual steno keyboard like the Uni to reach high speeds at maximum comfort.
Most keyboards are "staggered" which means that each row is slightly offset from the row below. The stagger originated from the original qwerty typewriter to prevent keys from hitting each other. It's a relic from the old machines that we still use to this day. Newer additions to the traditional keyboard like the numpad are ortholinear.
The Uni, comes with an ortholinear layout, which means that all the keys are in a grid.
This ortholinear layout allows you to easily press chords with one finger like "HR" on the left or "PB" on the right. This makes it a lot easier to write words in steno at faster speeds.