Bandwidth refers to data transfer. Each time a server transmits data or receives data from any other source, bandwidth is counted as the amount of data transferred. 

For example, if a file of 100 MB size is downloaded from a VPS to a PC or another server, 100 MB of bandwidth has been used. If the file is downloaded again, an additional 100 MB of bandwidth has been used. 

Bandwidth (data transfer) is allotted on a monthly basis, and resets to zero usage on the 1st of each month (regardless of service setup date).

Port speed refers to the maximum speed at which data can transfer, and may also be called "bandwidth speed" or "transfer speed". SolVPS VPS instances are configured with a 1Gbit port by default, with up full 1000Mbps transfer speed possible based on connection quality (the sending/receiving server must also be connected at 1000Mbps or greater).