What on Earth is Sanctification?

It is sort of like the old fellow who was a drinker, a carouser, a liar and a user, but was soundly converted and he characterized his progress by saying,
“I ain’t what I oughter be –
and I ain’t what I’m gonner be –
but thank the Lord I ain’t what I useter be neither!”

Hebrews 10:10

¹⁰ By ^[a]this will we have been sanctified through the offering of the body of Jesus Christ once for all.