If you find your Mac is running slow, or the fans are kicking in when you don't appear to be doing anything too intensive, you can see if you can identify what's causing it. Activity Monitor in OS X and macOS shows you how your Mac's resources are being used.
Launch Activity Monitor from the Utilities folder to see current processes, and the resources they take up. The columns show you things such as the CPU usage of a process or the RAM it's taking up. If there's a process that's hogging resources and you're confident it's not needed, you can end it by selecting it, then clicking Quit Process.
If you're just curious about how system resources are being used, click the tabs (CPU, System Memory and so on) to see graphs of your usage over time.