1) Your mind is your memory store.
2) Si is a cognitive process that scans that memory store very rapidly and compares sensory information being received against that within your mind.
Si is not a database, it is not memory, it is a cognitive function. It looks at and compares sensory information against one though which is your memories.
It doesn't tell you if it is right or wrong however! It is introverted perception by functionality, just like Ni.
I took my quotes from cognitiveprocesses.com