Variability is a key feature and challenge of future energy systems, especially ones with emissions reduction targets. Higher variable renewables deployment, increasing electrification, and climate change impacts increase supply, demand, and price variability. These changes provide opportunities for technologies, markets, and policies to mitigate this variability but also pose difficulties for planners and policymakers. This article summarizes the sources and impacts of variability in deeply decarbonized electricity systems, approaches for managing it, implications for modeling, and emerging research needs. It aims to synthesize the main insights on variability from the literature for subject matter experts in a range of fields and consumers of model outputs. This primer is relevant not only to increasing the understanding of interconnected sociotechnical systems where variability is a distinguishing feature but also to highlighting research gaps where interdisciplinary collaborations are increasingly valuable.