Could someone tell me what is the grip angle

The grip angle is a quantity that captures, for many purposes, the complex
and subtle mechanics of a tyre. Most people call this quantity "slip angle."
I think this name is misleading because it suggests that a tyre works by
slipping and sliding. The truth is more complicated.
Near maximum loads, the contact patch is partly gripping and partly slipping.
The maximum net force a tyre can yield occurs at the threshold where
the tyre is still gripping but is just about to give way to total slipping.