T -- Total Depravity. Human beings are so affected by the negative consequences of original sin that they are incapable of being righteous, and are always and unchangeably sinful; human freedom is totally enslaved by sin so we can only choose evil.
U -- Unconditional Election. God chooses to save people unconditionally; that is, they are not chosen on the basis of their own merit.
L -- Limited Atonement. The sacrifice of Christ on the cross was for the purpose of saving the elect.
I -- Irresistible Grace. When God has chosen to save someone, He will.
P -- Perseverance of the Saints. Those people God chooses cannot lose their salvation; they will continue to believe. If they fall away, it will be only for a time.