The mean is the average of the data, which is the sum of all the observations divided by the number of observations.
For example, the wait times (in minutes) of five customers in a bank are: 3, 2, 4, 1, and 2. The mean waiting time is calculated as follows:
On average, a customer waits 2.4 minutes for service at the bank.
If the plot shows a systematic change in the data over time, then you should use caution when interpreting a summary statistic such as the mean. For example, if the data values increase over time, then the mean is likely to also increase over time.