Salt is not just a universal seasoning. It also has a powerful chemical ability to retain juices within fish and meat during cooking, provided you distribute the salt evenly throughout the meat at the proper concentration.
Slathering salt on the outside of a fish or a piece of meat doesn’t work very well, unless you want the distinctive flavor and firm, smooth texture of a cured meat, like corned beef or smoked salmon. At such high concentrations, salt actually causes the proteins in meat to fall apart.
The subtler effect of brining is more widely useful. Brining is the technique of soaking meat in a dilute salt solution until the dissolved salt permeates the muscle tissue. You’re shooting for a final concentration of about 0.5% salt throughout the meat—weak compared to curing. The challenge with brining is getting the meat deep in the interior to be just as salty as the meat on the outside. Unless you know what you’re doing, it’s easy to end up with a steep gradient of saltiness.
Modernist brining, akin to cooking sous vide, soaks the meat for long periods (up to 24 hours) in a solution having a salt concentration only slightly higher than that target of 0.5%. The risk of oversalting is eliminated. Brining fish prior to cooking will season it, firm it, and protect its delicate color.
– Adapted from Modernist Cuisine at Home