Psalm 90 is a psalm attributed to Moses, and it reflects on the brevity of human life in contrast to the eternal nature of God. Verse 12, "So teach us to count our days, that we may gain a heart of wisdom," is a powerful plea for understanding and perspective. Let's break it down:

"So teach us to count our days...": This is a request for God's guidance. "Counting our days" isn't simply about numbering the passing days. It's about recognizing the limited and precious nature of our time on Earth. It's acknowledging our mortality. It implies:

Awareness of Time's Fleeting Nature: Recognizing that life is finite and we don't have unlimited time.
Intentional Living: Being mindful of how we spend our time and not wasting it on trivialities.
Prioritization: Understanding what is truly important in life.

"...that we may gain a heart of wisdom.": This is the purpose behind understanding the brevity of life. "A heart of wisdom" refers to:

Discernment: The ability to make sound judgments and choose wisely.
Understanding: A deep comprehension of life's meaning and purpose, especially in light of God's plan.
Right Perspective: Seeing things from God's point of view, rather than being consumed by worldly concerns.
Moral Integrity: Living a life of righteousness and virtue.

In essence, the verse is a prayer asking God to:

Help us understand how short and valuable our lives are.
Grant us the wisdom to live each day with purpose and intention.
Guide us to prioritize what truly matters, aligning our lives with God's will.

In simpler terms, it means: "Help us realize how short life is, so we can make the most of it and live wisely."

The verse is a reminder that true wisdom isn't just intellectual knowledge, but a deep understanding of life's purpose and how to live it in a way that honors God and benefits others. By acknowledging our mortality and seeking God's guidance, we can develop a heart that is filled with wisdom and live a meaningful life.