T-splines are a type of spline that allows for more flexibility and control over the surface of a model compared to traditional splines or NURBS (Non-Uniform Rational B-Splines). They were developed to overcome some of the limitations of NURBS, such as difficulties in representing complex shapes with a reasonable number of control points.
Autodesk acquired T-Splines several years ago, eventually folding its technology into Fusion 360 and Alias. However, many power users remain loyal to the standalone plugin for Rhinoceros 5. t-splines - v.4.0.r11183 download
If you lack a license, the tech is now accessible via Fusion 360. To help you find what you need, What are T-Splines
Benefits of Using T-Splines v.4.0.r11183 Benefits of Using T-Splines v.4.0.r11183