Shoestring potatoes are very thin (julienne) potato strips that have been deep-fried until crisp. They are sold in "cans" and eaten as a snack like potato chips. The canned shoestrings were people in the 1950's and 1960's.
The term also is used to describe very thinly cut french fries which are available frozen. The fries are then heated in the oven or deep fried.