How to understand the world coordinates, screen coordinates and viewport coordinates?

beginners learn Unity , do not understand these concepts very well, such as ScreenToWorldPoint ScreenToViewportPoint and other methods are converted with what formula, their concepts I refer to the official documents, but there is no illustration, the text description is very abstract, do not understand. Which master can explain it in popular language? Thank you very much!

Unity2d unity3d
Mar.17,2021

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MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b3b253-2c263.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b3b253-2c263.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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