Full Idea
The crucial distinction is that in the B-series positions in time are unchanging. Positions in the A-series, in contrast, do change: what is now present was once future and will be past.
Gist of Idea
In the B-series, time-positions are unchanging; in the A-series they change (from future to present to past)
Source
Robin Le Poidevin (Past, Present and Future of Debate about Tense [1998], 1 (a))
Book Reference
'Questions of Time and Tense', ed/tr. Le Poidevin,R [OUP 2002], p.13
A Reaction
So does A-series time consist of a property which things gain and then lose, or a location which things enter and then leave? Neither analogy seems to throw much light.