First of all, you can not get a coupon for a Converter Box any more.
They have closed that, and will not be issuing any more coupons.
You will need a Converter Box, only if you have an older TV, which is not Digital.
As far as how a Converter Box works...
You must have an antenna (inside or outside) which picks up local Broadcast Channels.
You hook the Antenna to the Converter Box, then
The Converter Box to your TV.
Do a scan for available channels,
Then watch what is available at your location.
You should know what is available where you are.
In some instances, the cost of an Antenna, a Converter Box and labor for that, may not be justified by what you can receive,
so, check what you can get first.
In other words, you may be better off subscribing to/staying with a Cable/Satellite Service.