Full Idea
A-series expressions include words like 'today' and 'five weeks ago', and can be true at one time and false at another; B-series expressions are like 'simultaneously', and are always true, if true at all.
Clarification
These are two ways of seeing time
Gist of Idea
A-series expressions place things in time, and their truth varies; B-series is relative, and always true
Source
report of J.M.E. McTaggart (The Nature of Existence vol.2 [1927]) by E.J. Lowe - A Survey of Metaphysics p.308
Book Reference
Lowe,E.J.: 'A Survey of Metaphysics' [OUP 2002], p.308
A Reaction
A-series gives time separate existence, where B-series time is purely relational. Intuition favours the A-series, but how fast do events travel against this fixed background?