Activity Monitor is an application that allows you to monitor exactly what is going on with your computer at all times by showing you how much memory, cpu, network and disk space is currently being used. It can also show you which application is using the resources and it allows you to quickly kill a process that is going haywire or has stopped responding.
The handiest feature of all is that it can show the status in the Dock so that you can quickly see if something has gone wrong and prevent crashes. Open up Activity Monitor (in Utilities inside Applications), then select the type of information you want from the View, Dock Icon menu. To make the most of this we’d recommend setting Activity Monitor to automatically launch when you log in. This is done by right clicking the icon in the Dock and choosing Options, Open at Login.
Keeping Activity Monitor in the Dock also has all sorts of unexpected benefits, for example it is very handy if you feel that your computer is getting slow. Most Macs that are kept in good shape can live for many years and still be as fast as the day they were purchased. The key to this? Keep an eye on the memory usage.
If all the memory is being used up soon after booting up your computer macOS will start using the hard drive to give you more memory than what is physically available (this is called swapping). The catch is, swap memory is much much slower than real RAM memory so you will notice this by feeling that the computer gets much slower. The solution? Use Cocktail to purge inactive memory (System > Memory), try cleaning up the applications that run in the background or install more memory.
Activity Monitor can also help you debug other problems. Battery life is directly dependent on how much load there is on the processor so if the battery in your laptop doesn’t last as long as you expect, have a look at how the processor is being used. Generally, to get anywhere near Apple’s specified battery life you should be around 10-20% usage so if the processor is averaging more than that some process is draining your battery.