"Do not let the sun go down on your anger." What does that mean?

Thanks for the question.
Anger is a big issue for all of us.
In this verse from Eph 4:25-26, Paul reminds, us that anger if prolonged can lead to more sinfulness. He is telling us to work on quick ways of reconciliation, instead of holding it as a grudge and be unwilling to forgive.
The article below explains it well.