Poaching is a culinary technique that involves cooking something in liquid with a temperature ranging from 140°F to 180°F.
This compares with boiling, which happens at 212°F, and simmering, in which food is immersed in a cooking liquid with a temperature in the range between 180° and 205°F.
Remember that 212°F is the hottest temperature water can reach. Once it hits 212°F, it doesn't matter if your burner is cranked up as high as it can go or not.
That's as hot as it is going to get. A hotter burner only uses more energy.
The exception to this is when cooking in a pressure cooker or pressure steamer (although the latter is mainly found in commercial kitchens), since the pressure produces temperatures that exceed 212°F.
One of the points of poaching is is that, because of the relatively low temperature of the cooking liquid, it produces no agitation, which makes it ideal for cooking delicate or fragile items, such as eggs or fish. You might see small bubbles forming at the bottom of the pot, but none that rise to the surface.
But other protein items, such as chicken, are often prepared via poaching, and some vegetables can be poached too.
Another advantage of poaching is that poached items will turn out moist and tender, which is certainly desirable in the case of fish where it is easy for fish to dry out when prepared using other cooking methods.
And because poaching involves keeping the cooking liquid at a constant temperature — a fairly low one, at that — it makes it less likely that items will be overcooked. At least, it would take a lot longer to overcook.
The liquid for poaching is usually stock or water with seasonings and aromatic vegetables.
Fish and seafood is traditionally poached in a liquid called court bouillon.
You can buy a fish poacher, which is basically an oblong cooking vessel fitted with a tray, to accommodate a whole fish and make it easy to remove the cooked fish from the poaching liquid without breaking it. It's a good idea to use one that's made of stainless steel or copper, since aluminum will react with with any acidic ingredients in the poaching liquid (such as wine or citrus juice), imparting a metallic taste.
You can use any pot for poaching eggs, but it needs to be big enough to accommodate the number of eggs. If you're doing four or more at a time, you want a pot wide enough so that they don't crowd each other. A splash of vinegar in the water helps hold the eggs together. I like to crack each egg into a ramekin and then gently slide it down the side of the pot into the poaching liquid while giving the water a swirl. The whirlpool effect also helps produce a more compact egg.