What are "strands" in the novel, This Is How You Lose the Time War?

A "strand" is not a specific time or place, but a parallel universe that the two main characters can inhabit as soldiers in the time war. It is their job to ensure that things are proceeding as planned in the different universes so the future can end up positive for their organizations' ends. So, when the characters talk about being in a strand, they are in a specific parallel universe and time; there exist many different possible universes, or strands, that they can hop between.