Bandwidth or Traffic refers to how much data can pass within a given time period.
For hosting, this is usually monthly.
500GB of monthly bandwidth means that your account can transfer 500GB of data with a given month.
Port speed also plays a role in bandwidth. It is how much data can pass over a period of time. Usually when talking about port speed you’re talking about a period of seconds.
A 1gbps port speed (note, gbps means gigabits per second, not gigabytes … 8 bits are in 1 byte). A 1gbps port will max out at 324000000MB of transfer in a month (324TB). This is because 1gbps = 1000mbps = 125MB/s. There are 2592000 seconds in a month (30 x 24 x 60 x 60) and 125 x 2592000 = 324000000. This means that if you ran the port full for an entire month, you’d transfer 324TB of data. You cannot transfer any more than that in a month on a 1gbps port.
If you have a 40mbps home internet connection, then the most you can transfer in a 30 day month would be 12960000MB or 12.96TB. (This is also why cell phone companies throttle or deprioritize you on an unlimited plan, they can’t really let you run the connection full throttle for an entire month… but I’m getting off topic here).
Port speed generally refers more to capacity than overall monthly bandwidth usage. If a server has a 1gbps port, then it can handle more connection simultaneously than say a 100mbps port, or a 10mbps port. The same is also true for your home Internet connection.
If streaming a movie at home takes 10mbps and you are streaming 3 different movies, that’s 30mbps of your 40mbps connection. You only have 10mbps of bandwidth left to consume. If someone starts a video chat or starts downloading a large file, everybody’s connection will start to degrade. Then suddenly your streaming movie only has 7mbps to use, thus degrading the quality of that stream or causing connection issues.
In a web hosting environment, if 10 people are downloading a 3GB file from the server, if the server only has a 100mbps port speed, then it will only be able to offer that file to those 10 people at a speed of 10mbps. If an 11th person chimes in to download the file, then everyone’s speed will be less than 10mbps. Having a 1000mbps port speed just allows for greater capacity (especially since you likely won’t be hosting files that are 3GB in size, but a video file can be that large and if you’re streaming something, you’re also downloading it).
Bandwidth in the hosting industry is largely inconsequential. Unless you are running a large video site or something with very large files, you will very likely not use 10GB of bandwidth in a month, let alone 100GB or even 500GB.
Unlimited and unmetered are mostly marketing terms meant to influence your decision. But most web hosting providers care very little about the amount of bandwidth their servers consume because they have plenty to go around, so rather than limit you based on bandwidth usage, they just offer unlimited.
But just like with unlimited disk space (since there’s no such thing as an unlimited hard drive) there is always a limit even if it is marked as unlimited. The limitation will be based on the port speed, although most datacenters will stop you before you reach that upper limit.