Full disclosure: You need to read the whole verse.
The whole purpose of quietness is to spend time with God every day, listening to what he has to say.
Then you will live in relationship with him, and that’s where confidence and strength come into the picture. It’s from God, not from you.
This is what the Sovereign Lord,Isaiah 30:15, NLT
the Holy One of Israel, says:
“Only in returning to me
and resting in me will you be saved.
In quietness and confidence is your strength.
But you would have none of it.