LEED stands for "Leadership in Energy Efficient Design"
"LEED for Existing Buildings: Operations and Maintenance" (LEED-EBOM) is a LEED rating system developed for existing commercial and institutional buildings (especially office buildings). It was created by the USGBC. Here is the best overview of LEED-EBOM I have read to date.
Another great question is "What ISN'T LEED-EBOM?"
LEED for existing buildings is not a quick fix scheme for addressing the short-comings of a building. It is rather perscriptive, in a good way, about best practices and standards to use when assessing a building.
It isn't easy. If you have not been grounded in efficiency efforts in earnest as a matter of regular business practices in your facility for a few years then this is going to take effort.
It isn't impossible. What I view as one of the best components of LEED-EBOM is that it heavily gives credits to having and using specific management practices (integrated pest control, Cooling Tower Water Management, Solide waste management policy, etc). If you establish and follow these practices, not only do you get points towards your certification, not only are you making an impact on the environment around you, but you're also improving your building's financial bottomline and making it easier for you to adopt other more agressive beneficial practices.