What does the verse mean, “…To KNOW them which labour among you, and are over you in the Lord, and admonish you”? (1 Thessalonians 5:12)

The definition of this word “know,” according to Thayer's Greek Lexicon, (which is specific for this one verse) is the following:

“Hebraistically, to have regard for one, cherish, pay attention to: 1 Th. v. 12”

The next verse after this verse confirms this definition, “And to esteem them very highly in love for their work's sake.” (1 Thessalonians 5:13)

"And we beseech you, brethren, to know them which labour among you, and are over you in the Lord, and admonish you; and to esteem them very highly in love for their work's sake. And be at peace among yourselves." - 1 Thessalonians 5:12-13 KJV

Whom is this verse speaking of? It is speaking of pastors, the shepherds appointed by God to feed, lead, and guard the flock.

"And from Miletus he sent to Ephesus, and called the elders of the church. ... Take heed therefore unto yourselves, and to all the flock, over the which the Holy Ghost hath made you overseers, to feed the church of God, which he hath purchased with his own blood." - Acts 20:17, 28 KJV

"Let the elders that rule well be counted worthy of double honour, especially they who labour in the word and doctrine." - 1 Timothy 5:17 KJV

"Obey them that have the rule over you, and submit yourselves: for they watch for your souls, as they that must give account, that they may do it with joy, and not with grief: for that is unprofitable for you." - Hebrews 13:17 KJV

"The elders which are among you I exhort, who am also an elder, and a witness of the sufferings of Christ, and also a partaker of the glory that shall be revealed: feed the flock of God which is among you, taking the oversight thereof, not by constraint, but willingly; not for filthy lucre, but of a ready mind; neither as being lords over God's heritage, but being ensamples to the flock. And when the chief Shepherd shall appear, ye shall receive a crown of glory that fadeth not away." - 1 Peter 5:1-4 KJV

What doctrine do the elders labour in, according to the Bible? To begin with, they labour in preaching the gospel.

"Then Peter said unto them, Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost." - Acts 2:38 KJV

"For the promise is unto you, and to your children, and to all that are afar off, even as many as the Lord our God shall call. And with many other words did he testify and exhort, saying, Save yourselves from this untoward generation.

“Then they that gladly received his word were baptized: and the same day there were added unto them about three thousand souls. And they continued stedfastly in the apostles' doctrine and fellowship, and in breaking of bread, and in prayers. ... praising God, and having favour with all the people. And the Lord added to the church daily such as should be saved." - Acts 2:39-42, 47 KJV