ALARA is the acronym for As Low As Reasonably Achievable, a legal requirement (in the U.S.) that radiation exposures to people should be kept, well, as low as reasonably achievable. There can be some debate as to what is "reasonable", but certainly many circumstances can be agreed to by most folks. For example if one needs to do lengthy maintenance on an easily movable component which is in a high radiation area, it makes sense to relocate the component to an area of background radiation, do the maintenance, and then move the component back.
ALARA follows from the LNT (Linear, No Threshold) radiation dose model. Since every bit of radiation involves a positive, non-zero increment of risk, we want to reasonably control radiation doses in order to minimize the risk.
Now risk is a probabilistic concept. Just because you get some radiation exposure doesn't mean you WILL get cancer, nor does it mean you WILL NOT get cancer from the exposure. We can only express a probability with our current state of knowledge.
As an aside, people frequently misinterpret probabilities. If a meteorologist provides a prediction of a 30% chance of rain on a particular day and it does rain, many people erroneously conclude that the meteorologist was wrong. Nope. The prediction was not 0%! It was 30%, meaning it certainly might rain. If the meteorologist provides 3 days worth of 30% rain chance predictions, we rightly expect rain on one of those days, though we don't know which one. But it might not rain on any of the 3 days (probability = (0.7)^3 = .34 = 34%).
Driving safety tends to be more intuitive than radiation safety. A vast majority of people understand that driving involves a risk of an accident and that it is difficult to predict who/when/where an accident will happen. It is broadly just a risk per mile driven. Certain factors can increase or decrease the risk (like being under the influence or using lights at night).
So I frequently use driving accident risk as an analogy to the LNT model.
And frequently, someone (usually a pro-nuclear energy, LNT-denier) will comment that given that analogy, that ALARA is OVERLY BURDENSOME because when we drive we don't go as slow as reasonably achievable.
Just like "As low as reasonably achievable" is the basic radiation protection rule, it is essentially identical to the U.S.'s National Highway Transportation Administration's basic safety rule of driving:
"Basic Speed Rule. The
Basic Speed Rule requires vehicle operators to drive at a speed that is
reasonable and prudent. As a corollary to this rule, State laws usually provide
that "every person shall drive at a safe and appropriate speed when approaching
and crossing an intersection or railroad grade crossing, when approaching an
going around and curve, when approaching a hill crest, when traveling upon any
narrow or winding roadway, and when special hazards exist with respect to
pedestrians or other traffic or by reason of weather or highway conditions." See
Uniform Vehicle Code �11-801."
In other words, the speed limit or radiation limit is the maximum legal limit. The limit is a limit of last resort. If certain conditions exist neither limit may be appealed to. In the case of the speed limit, if the visibility is bad drivers must slow down. In the case of the radiation limit, if radiation levels are high workers mustn't slow down.
In the case of the speed limit, most of the time conditions (weather, road obstacles, etc.) allow the employment of the limit. In the case of the radiation limit, most of the time conditions (shielding, work relocation, etc.) prevent employment of the limit.
But both depend on reasonableness to minimize health risk.