The mannequin editor lets you edit two different types of parameters: 'real' mannequin parameters and motion parameters. Both are displayed as parameters in the mannequin editor but are quite different under the hood.
'Real' Mannequin Parameters
These are the parameters the game code uses to provide extra information to playing actions and procedural clips. Some examples are: a target position when aligning an entity in the world, a weight value when fading an animation in/out, or a sound parameter passed to the sound system.
All parameters simply have a name and a value. The value's type can be anything from a number over a position/orientation in space to even more complicated structures.
Only a standard location with name "TargetPos" can be previewed in the editor though by using the Params track in the Mannequin Previewer.
When a procedural clip relies on a specific parameter and specific parameter type it is documented along with the procedural clip, see List of Procedural Clip Types.
Motion parameters are the parameters that get passed along to parametric animation, the Blend Spaces.
You can preview how these motion parameters will influence your animation by adding keys on the track named Params in the Mannequin Previewer. (see also Mannequin Editor Tutorial 2 - Tags & Previewing)