That’s a relief.
I never know what that kid will do next, so I can't get peace of mind.
I feel so relieved now that I know my dad no longer has cancer.
When your heart is relaxed, how do you feel? What if this is after something startling happens? You would feel relief or peace of mind.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.