To sun something means to put it in the sun so it gets warm or dry.
Sunning something can make it feel nice and cozy.
Examples:
The cat sunned itself on the windowsill.
I like to sun my clothes to dry them faster.
She sunned her face at the beach.
The turtle sunned itself on the rock.
I sunned my back for hours at the pool.
The lizard sunned itself on the hot sand.