1. An assurance or reassurance to someone to remain calm, composed, or without worry about a particular situation
You can stay calm, everything will be fine.
Podes ficar tranquilo, tudo correrá bem.
2. A directive or permission given to someone to relax and not be anxious about something
Don't worry about the presentation, you can rest assured.
Não te preocupes com a apresentação, podes ficar tranquilo.