Images and files for the site are kept here.
Files belong to a site too
The media library is scoped to the active site like the rest of the content. If you cannot see a file you uploaded, it is usually under a different site.
Smaller versions are generated after upload
After an image is uploaded, several sizes are produced so pages load faster. This happens in the background.
The file is usable immediately, but the optimised versions may take a moment to appear. That delay is not an error.
Deleting a file cannot be undone
There is no trash here. Deleting a file removes its record, the file itself and every smaller version of it from storage. Unlike leads, contacts and tasks, there is no way to get it back.
And the references are not cleared either: pages or products that used it still point at it and will show a broken image.
Check where a file is used before deleting it — or replace it first.
Deleting a folder does not delete its files
This is the opposite of the above, and not something to worry about: removing a folder leaves the files in it alone and simply leaves them unfiled.
A folder is a way of organising, not a container whose contents go with it.
Write the alt text
Alt text is what is read when an image fails to load, and what screen readers announce. Search engines use it too.
Writing it at upload takes a few seconds; going back to add it to hundreds of files is work that never actually happens.