There is something wrong with the downloaded data, which was modified during the downloading process or some data was lost. MD5 is used here for data verification.

The typical application of MD5 is to generate fingerprints for messages (byte strings) to prevent them from being tampered with. For example, if you write a paragraph in a file named readme.txt and generate an MD5 value for this readme.txt and record it, then you can spread this file to others. If someone else modifies anything in the file, when you recalculate the MD5 of this file (the two MD5 values are different), you will find that. If there is a third-party certification authority, MD5 can also prevent the "denial" of the document author, which is the so-called digital signature application.