When something is stressless, it means it doesn't cause worry or tension.
If something is stressless, it is usually good because it means you can relax and not feel anxious.
Examples:
The park is a stressless place to play.
I like to wear stressless clothes at home.
She finds stressless activities to do on the weekend.
Eating ice cream is a stressless way to end the day.
A stressless vacation is what I need right now.
The cat's life seems very stressless.