A six o'clock step in football is a step performed by the quarterback where they turn hips towards their dominant hand and step straight back. It is coined the "six o'clock step" because the quarterback is moving in a straight line backward from point A to point B, like moving from the 12 to the 6 on a clock. Similar to a shuffle step, the quarterback turns toward his dominant hand, which is now partially protected because it is toward the inside of his body, and takes a few steps backward.
After receiving the football, quarterbacks perform a "drop back" which is when they take a few steps backward. Contrary to popular belief, these few steps back can actually allow the team to make a lot of progress forward. This allows the quarterback to see the field at a larger glance by getting an overview from a few yards backward. These few steps backward can also open up a new lane for a receiver to run and can move the ball further down the field, rather than if the quarterback immediately moved forward or stayed put.
Some say that it is a natural inclination for quarterbacks to move backwards after the ball is snapped. Since quite a few defenders are trying to sack him, a gut reaction could be to evade the defenders by moving backward while a defender is keeping him out of the quarterback's way.