A Keyword filter means that all Internet traffic goes through the servers of the censor who is scanning the content for 'bad words'. This dynamic filtering is true for most filters in schools, libraries and companies. If the site contains bad words it is blocked.
The person who is offering the blocked information could prevent the censoring by hide the content inside of images. For the user there is almost no difference but it is difficult for a computer program to read the text inside an image. Also SSL encrypted traffic (a URL starting with https://) can't be scanned easily. You can test which keywords are blocked on your connection with the little onlinetool kword. There you can enter the keywords you want to test an click on send. When you get the message "you entered [your word here]" in return everything is fine, but if you get an error message you know which words are blocked.