I need someone to explain to me like im 5 yrs old.

[SinSational]

GB is actual transfer
Mbps is rate of transfer
1Mbps = approx. 320GB's

if your server was pushing 10Mbps every second of every minute of every hour of every day for 1 month, then you would use approx. 3200GB's a month.

the reality is that your traffic spikes and falls throughout the day.
the benefit to having a plan based on Mbps is that your server could be doing 8Mbps for an hour, 2Mbps for an hour, 55Mbps for an hour, etc etc, but your billing is based on the average rate of transfer, usually measured in 5 minute intervals. and with 95th percentile billing, your host will remove the top 5 spikes of traffic.

if you are on a 10Mbps metered port, there is no way that you will get the full 3200GB of transfer unless your server is pinned at 10Mbps for the whole month. so it is best to have your server on a 100Mbps port to allow your traffic to spike when needed. as already mentioned, your server could do over 10Mbps for portions of every day and you could still fall under the 10Mbps average for the month.