Definition of a symbol $s_{NN}$ appearing in particle physics

1. What is the meaning of $s_{NN}$ in particle physics?

See e.g. here http://www4.rcf.bnl.gov/brahms/WWW/thesis/karabowicz_phd_thesis.pdf page 18 in the pdf.

1. What is its relation to energy?

I need a definition for a beginner in particle physics.

The symbol $s_{NN}$ is in OP's context of RHIC the Mandelstam $s$-variable in a Nucleus+Nucleus collision. The $s$-variable is also known as the square of the center-of-mass energy.