There are two reasons why you might find some words underlined. If they only appear in text that you've typed it's likely (as has been suggested) that your browser's spell-checker is simply pointing out your errors. (Right-click on a word to see the alternative spellings offered).
However if random words in web pages are being underlined (and/or highlighted) then your computer has picked up some malware (called 'Text Enhance'). To get rid of it, download, install and run the free version of Malwarebyte's Anti-Malware:
http://www.malwarebytes.org/products/malwarebytes_free/
(NB: During the installation process you'll be offered a trial version of the PRO version. Remove the tick to refuse the offer).
Chris