1. Third person singular present indicative of 'estar' (to be) + 'tranquilo' (calm, peaceful, quiet). Describes a state of being calm, peaceful, or free from worry.
Everything is fine, he is calm about the situation.
Tudo bem, ele está tranquilo com a situação.
2. Used as reassurance or to tell someone not to worry; an expression of confidence that everything will be okay.
Don't worry about it, everything is under control.
Não se preocupe, está tranquilo, tudo está sob controle.