I am sure that many of you have been told to be careful with your car or your new lawnmower to break-in the engine properly. You manage speed, vary the load, etc. until there are a certain number of miles or hours. The oil is changed early, etc.. Many mechanical devices need this period to run properly over the long term.
Electronic and electromechanical equipment has some of the same needs. The mechanism is different but the need is there. Capacitors have to form up, and more. Some equipment has this done at the factory. The equipment is put in a chamber that is hot and the gear is operated. This burn-in period can vary a lot depending upon the gear. One example I have heard more than once is a new phono cartridge. After it has been installed and you play the first couple of records the sound can be thin. You can hear it open up over 20 to 30 minutes. It is wonderful.
Here are a couple of links about burn-in that may be helpful to you: