Tangents describe the entry and exit of curve segments from a key. This menu operates on the shape of curve segments around selected keys. Note that these settings affect only existing animation curves segments’ tangents

Spline Tangents: Specifying a spline tangent creates an animation curve that is smooth between the key before and the key after the selected key. The tangents of the curve are co-linear (both at the same angle). This ensures that the animation curve smoothly enters and exits the key. When animating fluid movement, a spline tangent is an excellent place to start. 

Linear: Specifying a linear tangent creates an animation curve as a straight line joining two keys. If the in tangent type is linear, the curve segment before the key is a straight line. If the out-tangent type is linear, the curve segment after the key is a straight line. 

Clamped: Specifying a clamped tangent creates an animation curve that has the characteristics of linear and spline curves. The keys’ tangents will be splines unless the value of two adjacent keys is very close. In this case, the out tangent of the first key and the in tangent of the second key will be interpolated as linear.