groovy sort with comparator syntax

I am just getting my feet wet with gremlin. I understand that gremlin is based on groovy. I found the documentation here, but I am still not sure what the syntax means.

I am a bit confused as to how the syntax of sort with a comparator works:

m.sort{a,b -> a.value <=> b.value}

Could someone explain what all the different bits between the { and } mean?