It's the most basic symptom of stuttering. Blocking is when you try to say a word, but nothing comes out. It's a momentary lose of speech function for particular or range of phonetic sounds due neurological misfiring. It can be described as an extreme amount of resistance / muscle tension when you try to execute a speech sound that you already know how to execute or have successfully done so in the past. You're literally "blocked/denied" from saying that word for a duration of time that seems arbitrary and often times stutterers try to push through it, which can present itself as a repetition of words, like "p-p-p please can you p-p-p pass the jam". Usually people who stutter have a combinations of repetitions and blocks. Some have more blocks than repetitions though, increasing the severity level of their stammer. Repetitions though are just mini-blocks that have a very very short duration and low intensity of physical tension. i.e the block is very brief, lasting less than a second and it looks like one is tripping over their words.