The phrase 'timestream' is thrown about a lot in Science Fiction; as there is no governing body in SF, there is no definitive definition of the term.
The basic idea is that time is fluid, and the the flow of time, while unidirectional and more or less unstoppable, may be changed. A change in the events upstream will effect events downstream, and time may jump its banks and move into a completely new course if a clever scientist can only develop a sufficiently cunning machine.
The term timestream is also used in those stories in which there is more than one parallel series of events -- two or more branching streams of time. These are metaphorically tributaries in reverse, where time branched off in two or more different directions from a momentous event. It is not easy to make a distinction between this and the similar construct of parallel universes.