Psalm 37, in general, contrasts the fate of the righteous with the fate of the wicked. Verse 15, specifically, is part of that contrast. It's a poetic and prophetic statement about the eventual downfall of the wicked. Let's break it down:

"Their sword shall enter into their own heart": This is a vivid metaphor. It doesn't literally mean the wicked will stab themselves. Instead, it suggests that the wicked will be undone by their own violent and destructive actions. Their evil schemes, their cruelty, their attempts to harm others, will ultimately backfire on them and cause their own ruin. Think of it as karmic justice or poetic retribution. What they intended for others will be their own demise.

"Their bows shall be broken": The bow was a primary weapon in ancient warfare. To say their bows would be broken is to say their power and their means of attack will be rendered useless. They will no longer be able to harm or oppress. This symbolizes their inability to carry out their wicked plans and their complete loss of strength and influence. Their ability to cause damage and exert control is neutralized.

In summary, Psalm 37:15 is a promise that the wicked, despite their apparent power and success, will ultimately be defeated by their own wickedness. Their schemes will backfire, and their ability to harm will be taken away. It serves as a comfort and encouragement to the righteous, reminding them that justice will prevail in the end. It also works as a warning to the wicked.