to put flour, sugar etc through a sieve or similar container in order to remove large pieces
2 also sift through
to examine information, documents etc carefully in order to find something out or decide what is important and what is not:
Police are sifting through the evidence.
sift something ↔ outphrasal verb
sift something ↔ out from
It's hard to sift out the truth from the lies in this case.