If you are copying and pasting content from other sources such as Word, pdfs or other websites, sometimes the background code that manages the formatting can come with it. (eg bold will remain bold text and the headings are already large) You may not notice any problem, but sometimes you may find that you have problems if you want to reformat the text.
Moodle comes with a clear formatting button in the editor, this only clears formatting that Moodle doesn't recognise, which will often rectify any issues with formatting.
However if you want to start with completely unformatted text or if you are having issues with formatting, you can paste it into the text editor as plain text, there are a number of ways to do this
Paste as Plain text function in Chrome
If you are using Chrome then copy your text as normal from your source, click the cursor into the text editor where you want to paste the text, then right click and choose 'Paste as plain text'
Paste as Plain text function in Mac OS
Copy text as usual and then click cmd shift V
Notepad in Windows 10.
In the search windows box at the bottom of the screen, type notepad, then select 'Notepad desktop app'
Notepad will open, then paste your text into it, it will automatically remove any formatting at this point.
Then copy the unformatted text from Notepad, and paste it into your text editor in Moodle.