It is abbreviation of the phrase Virtual Machine.
In the Chrome JavaScript engine (called V8) each script has its own script ID.
Sometimes V8 has no information about the file name of a script, for example in the case of an eval. So devtools uses the text “VM” concatenated with the script ID as a title for these scripts.
Some sites may fetch many pieces of JavaScript code via XHR and eval it. If a developer wants to see the actual script name for these scripts she can use sourceURL. DevTools parses and uses it for titles, mapping etc.