Statistics levels determine the overall size of the performance data within vCenter Server. Based on your preferences, you can alter how long the data will be stored for as well as the individual metrics for different interval durations.

There are five interval durations when working with performance graphs:

• Real-time: This data is not stored in the database, but the individual ESXi host is queried when such data is requested. Every 20 seconds, the sample of data is retrieved.
• Last day: Real-time statistics are aggregated to 5-minute intervals and stored in the database.
• Last week: The last day's statistics are aggregated to 30-minute intervals and stored in the database.
• Last month: The last day's statistics are aggregated to 2-hour intervals and stored in the database.
• Last year: The last month's statistics are aggregated to 24-hour intervals and stored in the database.

For each duration, you can configure the statistic level from 1 to 4. Each statistic level contains a different set of performance counters. With Level 1, you get the most useful set of performance counters, and with Level 4 you get a complete set of all performance counters available in vCenter Server. Of course, the statistic level will affect the overall size of the performance data that will be stored, as shown in the following screenshot: 

For more information about statistic levels, feel free to check the official documentation at https://docs.vmware.com/en/VMware-vSphere/6.7/com.vmware.vsphere.vcenterhost.doc/GUID-25800DE4-68E5-41CC-82D9-8811E27924BC.html.