The Curve Tangent node outputs the direction that a curve points in at each control point, depending on the direction of the curve (which can be controlled with the Reverse Curve(カーブ反転)ノード). The output values are normalized vectors.
For NURBS and Bézier spline curves, keep in mind that the value retrieved from this node is the value at every control point, which may not correspond to the visible evaluated points. For example, a Bézier spline might have 48 evaluated points, but only four control points, if its resolution is 12. For NURBS splines the difference may be even more pronounced and the result may not be as expected. A Resample Curve(カーブリサンプル)ノード can be used to create a poly spline, where there is a control point for every evaluated point.