Act justly and love mercy
You have been told, O mortal, what is good,
and what the LORD requires of you:
Only to do justice and to love goodness,
and to walk humbly with your God.
Perhaps we want to be good, but are not sure if we are on the right path. God tells us what is good, and what it means to be good. We are to do justice and to love goodness. This leads us to walk humbly with the Lord.
Justice is to give each his own. So, if I ask somebody to mow my lawn, I should pay him for my services. Or, I have to respect the private property of other people. There is justice and balance in the transactions between us and other people. Someone who is mindful of justice respects and esteems others. This is not about crying out like a toddler: "that's not fair." Rather, it is about wanting to give to each person his due.
This is an important part of righteousness. We are not called to follow a series of moral regulations. This is not about checking off a box and proving our perfection. We should get emotionally involved, feeling an attraction for goodness. St. Augustine talks about three faculties of the human soul: intellect, will, and heart. With our intellect, we are able to discern what is good. With our will, we are able to choose it and decide to do it. But when our heart is on the same page, when we desire what is good, then it becomes much easier to do the good. We become truly righteous.
To be humble is to recognize our place in the world. We may be good and have a lot of talents, but God is always in charge. We walk humbly when we recognize that we are in His hands. This also gives us confidence and peace, knowing that someone powerful is taking care of us.
When God tells us to act justly and love mercy, this is not some moralizing discourse to make us behave. Rather, it is opening a door to the happiness and peace that comes from acting according to our purpose in life.