From the Latin prior ("previous"), the priority refers to the precedence of something with respect to something else, whether it is in time or in order. That or that which has priority is found first in comparison with other people or things.
Taking into consideration that priority is in the first place before all things is a point of reference to understand how or what condition God has in our lives.
When we refer to the fact that God must have the first place in the life of the believer we are referring to the place he occupies in our actions, I refer in this case to the personal relationship with Him to what He truly means to us.
We ask ourselves how is the place that we are placing God in our administration of time in what we do, we think, if we always seek to please, respect, obey and exalt him at all times.
All these things are important when we reflect on what He means to each of us
Source
The book of Mark 12:30 tells us the following regarding the place we give to God Mark 12:30
And you shall love the Lord your God with all your heart, and with all your soul, and with all your mind, and with all your strength. This is the main commandment.
This is the main commandment that we apply to our life when we prioritize that God is first than everything we have in this world.
It is for this reason that having God in the first place is giving Him the preeminence in the most intimate part of our being. The psalmist David expressed it very well in chapter 119: 14-16
I have enjoyed myself in the way of your testimonies
More than all wealth.
In your commandments I will meditate;
I will consider your ways.
I will rejoice in your statutes;
I will not forget your words.
When we have God in the first place we keep his commandments out of conviction we obey him we praise him and we adore whatever the circumstance.