Please explain History Monoid in Simpler Terms

I tried to read the History monoid but couldn't wrap my head around it. Could somebody please explain it in simpler terms?

Thank you

Refere开发者_如何学Gonce: http://en.wikipedia.org/wiki/History_monoid

---

The history monoid is the set of possible sequences of primitive actions in the threads, taking into account synchronization primitives which occur in more than one thread simultaneously.

Actually it is not just a set but a monoid, which means that you can concatenate the sequences to get a new sequence in the monoid, and there is a neutral element, the empty sequence.