To cut food into long skinny "matchstick" size strips. The strips are usually about 1/8-inch-thick all around.
You'll find that most references to julienne will be about vegetables (such as potatoes or carrots). However the term is sometimes used for meat or fish (especially in regards to stir fry).
To julienne potatoes: first cut them into 1/8-inch-thick slices. Stack slices and cut them into 1/8-inch-thick strips.