Computer science ultimately boils down to the study of how to efficiently compute a solution to a given problem.
From here the field has a bunch of different areas that investigate aspects of this including things such as how difficult a problem is to solve, what kind of hardware is required to be able to compute a solution, the efficiency of an algorithm to solve the problem, data structures which make solving the problem more efficient, etc.
But ultimately the field is the study of efficiently in computing a solution to problems.
From here the field has a bunch of different areas that investigate aspects of this including things such as how difficult a problem is to solve, what kind of hardware is required to be able to compute a solution, the efficiency of an algorithm to solve the problem, data structures which make solving the problem more efficient, etc.
But ultimately the field is the study of efficiently in computing a solution to problems.