Paul is giving a series of commands as he ends 1 Thessalonians and here he tells them not to stop the work of the Holy Spirit in their lives, that is quenching the Spirit. It is the picture of throwing water on a fire. Apparently during this time when the word of God (The Bible) had not been completed they still had prophets speaking new revelation and the Thessalonians were apparently frowning on this happening. They were to allow the prophecies, but to do what it says in 1 Corinthians 14:29 Let two or three prophets speak, and let the others judge. This was to make sure they were not false prophets. They should retain everything that passed the test. What did not they should reject along with all other kinds of evil. This is true today, when we hear false teaching or people who teach or believe wrong things about the Bible. We ought to know the Bible so well that we can immediately recognize false teachings and lies about what it says. Can we do this?
1 Thessalonians 5:19-21 Do not quench the Spirit; do not despise prophetic utterances, but examine everything carefully; hold fast to that which is good;