Crackle finishes and methods are all about copying the original craquelure look of old paintings, and it's caused by the different drying rates of the various layers of paint and varnish.
There are a lot of different ways to achieve this look, from Elmers glue spread between layers of paint, to using cracked eggshells glued on and painted. This is a really old technique, reinvented for a new look.
There are many different crackle finish paint-on finishes which work in various degrees of success.
Generally, letting it almost completely dry, then painting on a thick layer of top paint gives you bigger cracks.
More diluted paint produces finer lines. It depends on the look you like, which type you use.
There are lots of things that can be improved by using a crackle finish; old furniture, rustic crafts like painted board boxes with twiggy or found object handles, picture frames, hook boards or any number of thrift store finds.
It's one of the best ways to make something look really old and vintage and add that aged and revered look to your home decor.