Markan Sandwich stories, also known as "intercalations", are a composition technique the Markan Evangelist adopts to develop a dialectic, interpretive relationships between two different narratives.
One story (the "bread") acts as a framing narrative around another narrative (the "meat or veggies"), with the two stories mutually enhancing and playing off of one another. Some examples of Markan sandwhich stories include:
Mark 5:21-43 - An outer/inner story comprised of Jairus' daughter and the woman with an ailment
Mark 6:7-30 - An outer/inner story comprised of the sending of the disciples and the trial and execution of John the Baptist
Mark 11:12-21 - An outer/inner story comprised of Jesus cursing a fig tree and the episode of turning over the money changing tables