The elements of a system refer to "the stuff" or the individual parts that make up a system. Elements tend to be the most visible and easily recognized components of a system and are often the things that we first notice or that most capture our attention. In basketball, for example, the "elements" might include all of the things that are used to play the game: the players, their shoes, uniforms, coaches, score board, fans, the ball, the court, the hoops, etc.
From a systems thinking perspective, elements are the least crucial determiners of a system's behaviour. This is because changing elements does not tend to create a significant change in the nature of a system. What would happen if we swapped out players on the basketball team, or changed the uniforms, shoes, or even the type of ball, but nothing else? While a team might perform a little bit better or worse, we would still recognize that we were watching a game of basketball.
Thus, while changing elements does not typically result in a significant change to a system, elements are the things we tend to notice and focus on the most. Systems thinking is about looking deeper — beyond the players to the rules of the game.