If you read the specified help section (
:h W11) you'll get some details on what's going on. In short, the file from which you loaded the current buffer has changed. Specifically, its timestamp and content are different
Without some extra steps (below), you can only keep one version. To keep what you're currently looking at press Enter.
L will load whatever is in the file on disk losing any changes you've made in the buffer since your last write.
You could hit Enter, then
:w to a different filename, then do
:e again. That way you'll preserve both versions.
Note that the triggering and/or display of this warning is particular to certain configurations. For example, with
'hidden' set you can get the warning if have unsaved changes in the buffer and then you move out of the buffer and back in. Without
'hidden' you won't see it because you'll get a warning about your unsaved changes and won't be able to leave the buffer without saving. Another example is
'autoread': if it's enabled you will not encounter the conditions that trigger the warning. And one more example is
'buftype' which must be empty for the warning to appear. (It is empty for regular buffers but things like popups, terminals, quickfix windows, etc. set it to something.)