A young John Connor is killed early on in the movie by a Terminator.
The Terminator that killed John decides since it has killed it's primary target, it has fulfilled it's mission & now no longer serves a purpose; ends up being a recluse, staying in a cabin out in the woods, growing old.
Sarah Connor has become a kind of Terminator-hunter, tracking them thru GPS coordinates, hunting them down & destroying them.
The Terminator in the woods is the one sending Sarah the GPS locations of these other Terminators.
In this timeline, the hope of the future & leader of the Resistance is no longer John Connor; it's the girl the human/hybrid terminator is protecting.
The artificial intelligence that threatens humankind is no longer Skynet; it's now called Legion.