It is knowing that all things are not good but that God will use all things for your good.
It is knowing that your relationship with God does not depend on how you feel but in what Jesus did on the cross.
“I know what it is to be in need, and I know what it is to have plenty. I have learned the secret of being content in any and every situation, whether well fed or hungry, whether living in plenty or in want. I can do all this through him who gives me strength.” (Philippians 4:12-13 NIV)
Being content is a choice, not a feeling.
It is knowing that no matter what life hands you, Christ is in you and He will give you the strength to handle whatever life brings.