Lost some times in terminology.
This first picture explains your Visa, on Entry in Thailand the Immigration Officer put a NEW stamp (second picture) in your passport with text: Admitted until and a date. This date is the day you have to leave Thailand or apply for an Extension of Stay at local Immigration Office. This Extension of Stay will be added to the Admitted until day stamped in your passport, this means you do not have to wait till the last day and you will not loose days by going early.
TLDR : Answer Summary
When you enter Thailand, the Immigration Officer stamps your passport with 'Admitted until' followed by a date. This date indicates when you need to leave the country or apply for an Extension of Stay. If you choose to apply for this extension, it will update your stay based on the original stamp, allowing you to do so without risking losing days.