At their core, a Blueprint Spline Component is just a path for you to define and use positional data. You can use it to move Actors (or other Components) around the world, or place a series of Actors (or other Components) along the spline. They are fully editable in the Blueprint Viewport and in the Level Editor, with the ability to add/remove/duplicate Spline Points, change their tangent types, and even animate them on tick. Further, they are also editable using the Blueprint Construction Script, taking in edits made in the Blueprint Viewport or Level Editor, and mofifying them further.
Blueprint Spline Mesh Components have a completely different use case. These deform a single Static Mesh along a two point spline. You cannot add more Spline Points to a Blueprint Spline Mesh Component, but the two points are completely controllable through Blueprints.
While their use cases are very different, they share the same procedures for adding them to a Blueprint, and use the same editing tools.