Blessed are the poor in spirit for theirs is the kingdom of heaven.
To be "poor in spirit" is the opposite of being self-confident or self-reliant, especially in any spiritual sense. The poor in spirit recognize they are incapable of providing for themselves by their own strength, goodness, or righteousness. They know themselves to be spiritually bankrupt of true goodness. They cannot hope to bargain or earn their way into the kingdom of heaven. Jesus says, though, that they are blessed because the kingdom of heaven is theirs already. In other words, admission that one does not deserve a place in God's kingdom is a requirement for entrance into that kingdom. This is the opposite of assuming one has earned citizenship by his own merit.
Blessed are those that mourn, for they will be comforted.
Those who mourn, by definition, are not happy. Jesus wants His followers to understand that those who experience mourning are not hopeless.
Those who mourn may do so because of their own sin or because of the sin of Israel. Repentance involves sadness as we recognize and confess our sinful selfishness. Those who mourn while repenting of their sin are blessed, however, because the kingdom of heaven will come; they will be rescued. Not all commentators agree that Jesus is referring only to mourning that comes with repentance from sin. Much sadness in life, of course, is not the result of sin. It comes from living on a planet ruled by death. Of all who mourn, those in Christ are blessed because they will be comforted by God in the here and now (2 Corinthians 1:3–7) and free from mourning for eternity (Revelation 21:4).
Blessed are the meek, for they will inherit the earth
The word meek does not mean "weak," though that's how many misinterpret it. The word is best understood as "gentleness," or a refusal to take control by brute force or manipulation. Meekness, from a Christian perspective, is about faith. It is about trusting God to win the battle in the end instead of going to extremes to win the battle on our own terms. The concept is often referred to as "strength under control," rather than "strength seeking control."
It should be said that true meekness takes courage; we must fight with ourselves to trust God's plan and His character.
Blessed are those who hunger and thirst for righteousness, for they will be filled.