Hello @DheerajKumarn! Welcome to the Community.
If you're on a postpaid service, Fido Roam is automatically part of your plan and can only be deactivated on our end. If you do so however, you may still incur roaming fees at the pay-per-use rates.
With this feature, you can use everything that's included in your plan for $7 / day while in the US.
If you wish to avoid any roaming fees, you have to make sure that your mobile/roaming data is turned off, to not send texts and to not make/receive phone calls.
You can also put your phone on airplane mode, turn off the device or remove the SIM.
For more info, you can visit this page.
You'll also find the pay-per-use use rates for prepaid and if you deactivate Fido Roam.
Hope this helps.