With this era of advanced analytics producing a deep and complex lexicon, we can lose sight of what some of these new metrics and concepts aim to capture. Usage rate is one metric we often hear spouted from basketball analysts (ahem) that could use more clarity and context.
The simple explanation is that usage rate is an estimate of the percentage of team plays a player is involved in while on the court. It's true, the metric doesn't measure cheering from the bench, but it does weigh field goal attempts, free throw attempts, minutes and turnovers into a complex formula that aims to capture how often a player is directly influencing one of three end points for each play (turnover, free throw or field goal) for his team.