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Microsoft's Windows operating system uses a thumbnail cache for thumbnail icons used in Explorer to speed up the display of icons when a user opens a folder with cached items.

Microsoft changed how the thumbnail cache works in the Fall Creators Update for Windows 10. Starting with that release, Windows 10 clears the thumbnail cache automatically using a feature called Automatic Maintenance.

Automatic Maintenance

Automatic Maintenance performs a large number of tasks, from synchronizing the time and maintaining Windows Defender to defragmentation of drives and clean-up operations.

Tip: Open PowerShell and run the command Get-ScheduledTask | ? {$_.Settings.MaintenanceSettings} | Out-GridView to display all Automatic Maintenance tasks and the status of each in a listing.

One such task is called SilentCleanup. The task launches the built-in Disk Cleanup tool using the parameter /autoclean which makes the tool read Registry values to determine what to clean.

Cleaning the cache may free up storage on the device but it comes at the cost of having to renew the cache when File Explorer is used. 
Windows 10 users who keep lots of photos in a single folder may experience issues when those folders are opened in File Explorer as it may increase the CPU load for the time it takes to generate and cache the thumbnails.

You can visit the hidden folder %userprofile%\AppData\Local\Microsoft\Windows\Explorer in Explorer to display the cache.