Of course when you buy a couch, you have to find a way to get the old one out of the living room before you bring in the new one. Your brain is very similar: there is a clear mechanism for sorting memories into two categories, keep as permanent or discard and forget. We must clean out the “active” space to make room for new learning.
We perform our brain house-cleaning while we sleep, through a mechanism called the glymphatic system, which helps erase the memories we don’t want to keep.
Memories we DO want to keep we tag for remembering in a simple way: by paying attention and focusing on them!
The website FastCompany has a nice description of the details of cerebral housecleaning, reminding us to get good sleep and to focus on the parts of our brain we want to hold on to.