Word guessing refers to a method of reading where a beginner reader doesn't know what a word is in a sentence, so he/she guesses what the word is and reads the rest of the sentence to confirm their guess, e.g. The fox jumped over the dog. If you didn't know the word "jumped" then you might read it as: "joo-mp-ed" then you would read the rest of the sentence and realise that it was actually "jumped".
References
Word guessing Wikipedia(Text) CC BY-SA