What do they mean?
They are all referring to a standard, called ECMAScript.
Why this weird name? Ecma International is a Swiss standards association who is in charge of defining international standards.
This press release by Netscape and Sun Microsystems (the maker of Java) might help figure out the name choice, which might include legal and branding issues by Microsoft which was in the committee, according to Wikipedia.
The current ECMAScript version as of time of writing (Sep 2017) is ES2017, AKA ES8
It was released in June 2017.
Every standard version proposal must go through various stages, which are explained here.
I found it puzzling why sometimes an ES version is referenced by edition number and sometimes by year, and I am confused by the year by chance being -1 on the number, which adds to the general confusion around JS/ES 😄
Before ES2015, ECMAScript specifications were commonly called by their edition. So ES5 is the official name for the ECMAScript specification update published in 2009.
Why does this happen? During the process that led to ES2015, the name was changed from ES6 to ES2015, but since this was done late, people still referenced it as ES6, and the community has not left the edition naming behind - the world is still calling ES releases by edition number.
This table should clear things a bit:
So at the time of writing, ES8 has been released, and ES.Next is ES9.