A reset is basically taking a few steps back in training after you run into a stall or another issue which cannot be resolved by increasing the work (weight, sets, reps, or additional lifts). The steps back will give your body a chance to "recover" and adapt to the previously high workload. You'll eventually work back up to the previous stall and if correct programming is used, continue to push past it for some time before another reset or change is needed.
Most resets are not planned, when a weight/reps/sets cannot be completed for 1-3 sessions a reset is probably in order. I say 1-3 because some people just have bad days (hard day at work, bad sleep/eats, stress at home...). Another reason is because at some point a person will not be able to progress from session to session and may only be able to progress every other session.
One way to avoid a reset and continue to push onward is to change things up slightly. If for example your session's goal is to hit 3x5x185 but have only been able to get 2x5, 1x3/4 for sometime, change to 5x3x185 and continue to push weight. The volume will be the same but the smaller sets will be much easier to handle. This should buy you more time for progression before a reset is necessary.