What is Letting Go?

Recently had a break up with my best friend of 5 years. More like she broke up with me out of nowhere. Yup, it was soul shattering for me, almost 15 days to the incident. Cried and grieved every day, my other friends telling me to let her go, don't hold onto her.

My question is- WHAT DO WE MEAN WHEN WE SAY "TO LET GO"? What is letting go actually, is this erasure of her memories or suppressing her existence or anything else?