It’s your choice. Many of us choose to wear it under our clothes because it’s a private devotion and we don’t show off our prayers. There are also jobs where you’re either not allowed to wear it out in the open, or it would be dangerous to have something dangling off you (gets caught in machinery, catches fire over a stove etc).
Some people choose to wear them showing because they want to bear witness or for some other reason (like it itches when worn under clothes).
If you’re not a member of the secular order then it’s up to you whether you wear it under or over your clothes.
