I quote from the official https://www.torproject.org/docs/tor-doc-relay.html
"If you have at least 2 megabits/s for both upload and download, please help out Tor by configuring your Tor to be a relay too"
What happens when i configure my tor to be a relay? Does it mean that if someone near my place/area who uses tor his/her anonymous data go through my network?
If so even if it is encrypted can I just see what happens? What data packets are being sent. How my badnwidth is used? Anything?
A relay could be used by anyone, anywhere in the world. They don't have to be physically near you.
You can get relay statistics from https://atlas.torproject.org/
If you are an Exit then peoples traffic will be visible, at least in the form it was sent into the Tor network (e.g. if they're using HTTPS youd only see their TLS tunnel, not the traffic inside of it). Actually looking at this is discouraged and violates wiretapping laws in many parts of the world. (Some not-so-bright Swedish script kiddy claimed to have "hacked Tor" by running an exit and intercepting peoples traffic, he was subsequently charged under Swedish law).
By default if you are a relay, you will act as an Exit. If you do not wish to act as an exit point in the Tor network (where clients traffic is sent to it's intended destination) then make absolutely sure that you specify ExitPolicy reject *:* configuration line that the guide you linked specifies.
Otherwise you have no visibility of the cells and no means of knowing what they're used for. That's the point of an anonymity network.
