The Greek word for 'mourn' is penthos; it literally means 'grief.' This word is a strengthened word from a root that literally means
'to experience suffering.' So, here, God is promising blessing to a category of people whose condition of mourning is an expression of suffering grief.
With that in mind, take a moment to examine some categories of mourning, to see where you may fit. First, the Scriptures indicate a category of individuals who mourn improperly.
John Macarthur relates an Arab proverb, 'all sunshine makes a desert.' And one of my favorite Jewish proverbs comforts, 'what soap does for the body, tears do for the soul.' Sorrow serves expresses purposes in our life, and you should embrace those experiences. Mourning is a time for learning, for growing, for trusting, for remembering, for healing, and for hoping.
The Scriptures share many experiences of appropriate mourning. Abraham grieved over his loss of Sarah. Not from a lack of faith, but from an awareness of his loss. David grieved over the assault of his pagan enemies. The father of the demon-possessed son mourned over his son, whom he brought to Jesus. And Jesus Himself grieved deeply for the sins of Israel, and for the deep loss He had for Lazarus, even though He knew that Lazarus would soon rise from the dead and dine with Him.
You may find it interesting that these two categories of mourning, 'proper or improper,' while the most prevalent in the world, are not the sort of mourning addressed at all in this beatitude. The sorrow mentioned here that is the recipient of blessing is to those who know the reality of godly mourning.
Look at how this mourning is defined in the Bible:
For godly sorrow produces repentance to salvation, not to be regretted; but the sorrow of the world produces death. For observe this very thing, that you sorrowed in a godly manner: What diligence it produced in you, what clearing of yourselves, what indignation, what fear, what vehement desire, what zeal, what vindication! In all things you proved yourselves to be clear in this matter. (2 Cor. 10:11)
This means is that the only sorrow that leads to spiritual life is godly sorrow, which is sorrow over sin that leads to repentance. This doesn't invalidate or disregard other legitimate or appropriate sorrow, but in terms of the spirit life, the most import sorrow is sorrow that is linked repentance over sin.
Godly sorrow helps you understand the connection to mourning that is presented here in the beatitudes. Remember, to receive entry into the kingdom of heaven (both here and now, as well as in the hereafter), you must be poor in spirit. That means that you must continually and consistently remember that you come to Christ empty-handed. You bring nothing to the table. You add nothing to the equation. And you should be ever mindful of that position. Paul recalls this in Romans 7:18, when he wrote, 'nothing good dwells in us, that is, in flesh.'
The only way to experience godly sorrow is to be poor in spirit. Likewise, those who are poor in spirit become those who mourn. You have biblical precedence for this theological connection. Job, according to the Bible, was a godly, upright man, who had nothing that could be held against him. But Job had to lose everything before he came to the point of godly sorrow. Look what he declared: "I have heard of You by the hearing of the ear, but now my eye sees You. Therefore I abhor myself, and repent in dust and ashes (Job 42:5-6)." David expressed godly sorrow for his sin against Bathsheba: 'For I acknowledge my transgressions, And my sin is ever before me. Against You, You only, have I sinned, And done this evil in Your sight That You may be found just when You speak, And blameless when You judge (Ps. 51:3-4).' God loves for you to behave morally and to live righteously, but even more so, He desires you to be humble (poor in spirit) and contrite (in godly mourning).