"Nesting" (in this case) refers to the act of encapsulating one thing within another of a similar type (eg, unborn babies).
Parantheses are round brackets, often used for inserting non-essential or explanatory information into written text. Some people also wish to add notes to their original note, hence the nested parantheses.
Many people find nested brackets incredibly annoying. Nesting paranthesis in plain text is an occupational hazard of C/C++ Programming