The intention is to detect those particular components that would fail as a result of the initial, high-failure rate portion of the bathtub curve of component reliability. If the burn-in period is made sufficiently long (and, perhaps, artificially stressful), the system can then be trusted to be mostly free of further early failures once the burn-in process is complete.
A precondition for a successful burn-in is a bathtub-like failure rate, that is, there are noticeable early failures with a decreasing failure rate following that period. By stressing all devices for a certain burn-in time the devices with the highest failure rate fail first and can be taken out of the cohort. The devices that survive the stress have a later position in the bathtub curve (with an appropriately lower ongoing failure rate).
Thus by applying a burn-in, early in-use system failures can be avoided at the expense (tradeoff) of a reduced yield caused by the burn-in process.
When the equivalent lifetime of the stress is extended into the increasing part of the bathtub-like failure-rate curve, the effect of the burn-in is a reduction of product lifetime. In a mature production it is not easy to determine whether there is a decreasing failure rate. To determine the failure time distribution for a very low percentage of the production, one would have to destroy a very large number of devices.
When possible, it is better to eliminate the root cause of early failures than doing a burn-in. Because of this, a process that initially uses burn-in may eventually phase it out as the various root causes for failures are identified and eliminated.
For electronic components, burn-in is frequently conducted at elevated temperature and perhaps elevated voltage. This process may also be called heat soaking. The components may be under continuous test or simply tested at the end of the burn-in period.
There is another use of the term by some audiophiles, who leave new audio equipment turned on for multiple days or weeks, to get the components to achieve optimal performance. However, many debates arise about the beneficial effects of this practice.