What is grace ? My first encounter with the word grace was in the verse Ephesians 2:8,9 For it is by grace you have been saved through faith-- and this is not from yourselves, it is the gift of God--not by works so that no one can boast. Another way of expressing this is heaven is a free gift or going to heaven is by grace. Grace is God's voluntary and loving favor to those He saves. By the way it took a while for me to accept that heaven or salvation is a free gift. Since the Bible also says in Romans 3:23 that all have sinned and fall short of the glory of God, that makes all of us as sinners . Yet as sinners we are still offered or given salvation.
Another use of grace is in 2 Corinthians 12:9 where God told Paul " My grace is sufficient for you, for my power is made perfect in weakness." ( under study for now).
Back to grace, let me try to picture it this way. Let say God is a bank and we are robbers or active robbers (or sinners according to Romans 3:23). Grace is like the banker calling the robbers , to admit and confess their /our crime and we will be forgiven and be allowed to freely withdraw money from the bank even when we do not have a deposit.
For us humans this does not make sense, but that is how grace is , it is God's unmerited favor to sinners or free gift to sinners. Do we remain robbers? not anymore because we will be regenerated or sanctified.(another topic).