BSNL saw a gap in India. While people had high-speed broadband connectivity at homes with Wi-Fi, they couldn't access it outside their homes/office.
But with BSNL Wi-Fi Roaming service, even when you are outside, you can connect with BSNL's Wi-Fi network (if it is available in your area/location) and continue using your plan.
If you purchased a 100 Mbps plan for your home, then that is what you will get outside your home as well.
If you are a BSNL FTTH customer, then you will have to register for this. Just simply go to this link - https ://portal.bsnl.in/ftth/wifiroaming (I have broken the link, you can copy paste and remove the spaces in it).