Put most simply, if you think of a file as a necklace of a string of data blocks, you can have something that's very irregular and widely spaced as you go along the strand (fragmented into islands of data chunks) or that are all cheek by jowl, making the necklace more like a solid piece of skillfully bent metal (unfragmented).
If you are using a Windows 10 machine disk defragmentation should already be set to occur automatically if your computer uses a conventional HDD. To check:
Control Panel, Administrative Tools, Task Scheduler, then Expand Task Scheduler Library, followed by Microsoft, followed by Windows, then select the Defrag entry. I will be shocked if it's not already set to run with the last run time shown along with the exit status (which should be 0x0). I haven't been doing manual defrags on a Windows machine for quite some time since it began being scheduled as a part of the standard Windows configuration.
If you want to know if your laptop is overheating then go to http://www.cpu-world.com/, then enter your CPU (e.g., A12-9700, i5-4300) in the search box. If you don't know what your processor is, then open Settings,System, About Pane and look at what's shown in the Processor entry. In the search results returned you will generally have a value for Maximum Operating Temperature which is the highest temperature considered to be within normal limits for that processor. Most will not get anywhere near to that temperature for any extended period of time when they are not under a significant load. Sometimes, when under a really high stress load, they will hover near it and sometimes exceed it very briefly. This temperature is less than what is called the critical temperature at which most modern processors will start throttling their own performance and, if that doesn't cool things down, shutting themselves down immediately and ungracefully.
Brian AKA Bri the Tech Guy (website in my user profile) - Windows 10 Home, 64-Bit, Version 1803, Build 17134
. . . the presumption of innocence, while essential in the legal realm, does not mean the elimination of common sense outside it. The willing suspension of disbelief has its limits, or should.
~ Ruth Marcus, November 10, 2017, in Washington Post article, Bannon is right: It’s no coincidence The Post broke the Moore story