A model of computation is the definition of a set of operations allowed to compute a given algorithm (like a calculation, sorting, searching) and its respective costs. By assuming a certain model of computation, it is possible to measure the cost of computing the algorithm - its complexity. This cost is measured in the execution time and / memory space used to compute the algorithm. Models of computation are also used to analyze the limitations (if any) to compute some algorithms.
There are many models of computation, differing in the set of admissible operations and their computations cost. This chapter of BookOfProofs is used to introduce such models.