I've blogged briefly about letting go before. But really what I think of when I hear the phrase "letting go" pertains to the past; more specifically of letting go of an idea or memory of a person, which brings up the corresponding emotions. It means letting go of the things (emotions, fears, feuds, etc.) that are keeping you from not only reaching your full potential, and being successful and happy, but finding out who you really are as a person.
Letting go also brings to mind cleansing - not just the literal cleansing of your body, but of your life. Letting go of those people or those emotions that are holding you back, bringing too much negativity into your life, and distracting you from what you want to achieve.
For me, personally, I know there is still quite a bit of anger left over from my teenage years, mainly involving my father and stepmother. I know I'll need to fully deal with that eventually, but for the last few years it's been a slow realization process; really remembering things that upset me, things that I never dealt with at the time. It's the first step, but there's still more to do.
What are the things you need to let go of?