Disk Usage: This is the allotment of storage you have on your account. This would be your "hard drive".
Bandwidth: This is the amount of data transfer your site has had between your account and someone viewing your site.
MySQL Disk Usage: Similar to your disk usage. However, this is only tallied from data stored within a database.
CPU Usage: The amount the server has to work to display your webpage. Scripts such as WordPress tend to run high CPU usage.
Physical Memory Usage: This is your RAM "Random Access Memory". RAM is used to store temporary memory to help execute tasks on your website.
Number of Processes: As the name suggests, this is the number of processes running on your account.
I/O usage: Input / Output usage is how much disk activity your hosting account is using. This limit is in place to prevent one account from over-utilizing the disk, which would slow down or prevent other sites from working.
IOPS: This is the amount of IO operations per second your account is using.