How to verify if a file is valid
Open the terminal and point to the file you want to check.
sha256sum 2020-05-27-raspios-buster-lite-armhf.zip f5786604be4b41e292c5b3c711e2efa64b25a5b51869ea8313d58da0b46afc64
Once you have downloaded your file you can now check this number against the one on the site where you downloaded the file from. They should match. If they down then you should not install or run that file.
See the above number is the same as the number on the Raspberry PI wesite as the number produced by running sha256sum again the downloaded file. Which is the Raspberry PI operating system in this case. But the same principle applies for any files.
Aslo another Example
echo this is contract | sha256sum eec82bac6507ba7b32cd7c91df226c6cf21143431a2febda8be510c00603c885 -