A program is not one thing, it is a thing at a moment. Someone's saved file, the API you shipped last month, the build on the server in March, all point at a version that is not the one in front of you now. Versions are the names for those moments, and without them the question 'which one produced this?' has no answer.
About 1 min of reading
Name the moment.
The artifact
Before you run it, say out loud what it will print. Then open the answer.
The moving name
Before you run it, say out loud what it will print. Then open the answer.
Read the first example one line at a time. A line that prints shows what appears on screen; the rest set things up quietly.
The version has to be in the thing you ship: the file, the log line, the response header, the footer of the report. A version that lives only in your head is not a version.
A project with no versions yet starts at 0.1.0 or 1.0.0 and says which. The first release is the one that sets the convention, and it should be a deliberate choice, not an accident.
'It crashes on save' means nothing until you know the version that crashed. A version number in the report turns a complaint into a reproducible thing, and that is the first reason versions exist.
Major.minor.patch says what changes to expect: patch is fixes, minor adds backwards compatibility, major may break it. That promise is what lets a dependency range be written at all.
'Latest' is a moving name. Two people asking for latest in the same week can get different things, which is why a build pins an exact number and never a moving name.
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.