At the most basic level, evolution graphs show you trend lines for your metrics over time. You can always check which metrics are being shown in the evolution graph via labels positioned at the top left-hand side of the graph itself. If more than one metric is reflected in the graph, then each label will be colour coordinated with its corresponding line in the chart.
The x-axis (horizontal line and labels at the bottom) of an evolution graph represents the time period that the graph is focused on. Below this, you may see annotation icons if you use the annotations feature.
The y-axis (vertical line and labels on the left) represents the value of the primary plotted metric. If you compare more than one type of metric, for example, a visit count against a percentage or a pageview count against loading times, then the additional metric labels are shown on the opposite side.
The example screenshot above shows the count for pageviews on the left y-axis and average generation time on the right-hand y-axis. As the metrics for Pageviews and Unique Pageviews are numeric counts, you can reference the numbers on the left-hand side and as the Avg. generation time is measured in seconds, so you would reference that metric against that second count on the right-hand side.
For more detail on a specific plot point within the graph, hover your mouse over the circles within the trend line to reveal the specific values, such as the exact conversion value on that specific date in the example screenshot below.