I am trying to understand what is a Higgs bundle as defined in this paper by [Gukov and Pei](https://arxiv.org/abs/1608.01761).  They say it is a pair $(E, \Phi)$ 

* $E$ is a holomorphic principal $G^\mathbb{C}$ bundle 

* $\Phi \in H^0(\Sigma, \mathrm{ad}(E) \otimes K )$

Can anyone explain to me a little about this sheaf $\mathrm{ad}(E) \otimes K$ ?