The Splines package enables you to create and use splines as easily as you would any other object. Open the GameObject menu to create a spline, then add whatever components you want to use that spline’s path.
As an artist, this means you just need to learn one set of tools to draw roads onto your terrain, define camera paths, or extrude mesh shapes for level design. Even better, the spline you draw for your camera can be reused to place a path on the ground, to navigate characters, or anything else. Just add or swap out components as needed.
For developers, the Splines package provides a robust and standardized framework to build on. Create your own custom components or Unity Asset Store packages. For more information, see the Splines API documentation.
Source: Unity Technologies Blog