so that you may approve what is excellent, and so be pure and blameless for the day of Christ,In Philippians 1:9, Paul prays for abounding love, "more and more" for the Philippians. He then specifies that this love is in "knowledge and all discernment". Now he tells us a purpose...
(Philippians 1:10, ESV)
To "approve" means to test or examine. And the word that is translated "excellent" is the greek word διαφέρω (diaphérō) which is literally two words together, meaning to carry through. It is translated "excellent" because this word had the idea of the things that lasted, the things that would carry through, the things that were durable.
So, Paul is saying that abounding love in knowledge and discernment will enable you to figure out the lasting things of life. i.e. The things that really matter. The result is then that on judgment day, you will not be ashamed.