The network shows your bandwidth, including the download and upload speed. Then when your networks turns down, check this item to get where your speed has gone.
Memory shows you the general used memory, when your Mac runs slow; it may caused by the full usage of your memory. You can check this item first; you can optimize memory when free memory shows too little.
Disk shows you Hard Disk model, used space, free space wisely, and also read speed and write speed in real time.
CPU shows your current usage and CPU Type, it still shows the number of all running processes and treads.
The highlight of battery is that it can alert you when battery is too lower, this can avoid of sudden shutting down of your Mac. When you type a large number of files, and have not saved them all, this feature can definitely helps you a lot.
Sensor shows your CPU, GPU and Disk temperature. When one of them shows too high, it means you need to close your Mac to make it have a rest, because high temperature may cause your Mac crash, and limited the using age of your Mac.