What is this small tissue structure found in kidney?
This is a slice of an adrenal gland:



Source (Creative Commons Attribution-Noncommercial-Share Alike 3.0 License): http://histology.medicine.umich.edu/resources/endocrine-system 

These glands sit on top of the kidneys, typically under adipose tissue (which can also be seen in both your image and the image I included here). The most identifying feature of the adrenal gland is its clear separation into two parts: the outer "cortex" and inner "medulla", as labeled on the image.

The two parts have different endocrine functions: the cortex primarily produces steroid hormones, like aldosterone and cortisol, whereas the medulla makes catecholamines epinephrine and norepinephrine (also called adrenaline and noradrenaline). I believe the inset in your image is of the medulla.