If you copy text from the internet or another document and paste it into your document bad things can happen. The fonts and text colors may not be the same, the background could be a different color, etc.
A clipboard will appear just below the information you pasted. Click on the clipboard and select the second option, match destination formatting. Now your new information is formatted like the rest of your document.
If you regularly copy and paste, you may want to select the last option and set the default Paste option to Match Destination Formatting.
Don’t forget to use footnotes or cite your source if you are copying someone else’s text.