You can find the definition of the terms “elapsed” and “active” here under "Cycle Metrics. Elapsed is all the cycles that were counted for this unit, while active refers to the subset of these that processed data. You can also find more information in this recent post.
The throughput of the L1 cache is given in the metric l1tex__throughput.avg.pct_of_peak_sustained_active, please refer to the SpeedOfLight.section (GPU Speed Of Light Throughput), which shows the metrics associated with the high-level throughputs of many important HW units.