In a very, very, short nutshell:
In the pre-existence we were begotten spirit children of heavenly parents. In order to become like God we were sent to this earth through birth from earthly parents and a veil was put over our minds as our body was received. The reason we are here is to prove our worthiness to God and gain a resurrected and purified body. To do that, we serve others and endure trials. We do this by actual service, discovering truth and sharing it, etc... and it's all done with love or it is completely pointless. When we leave this mortal life there are differing degrees of glory to satisfy the demands of justice and mercy. Jesus, being the mediator, made it so we don't all go to outer darkness (or actual hell). He literally saved us. However, our part is still to prove our worthiness by our good works and faith.
"If a nation expects to be ignorant and free, in a state of civilization, it expects what never was and never will be."