See first : How to sear meat in a pan and let it rest
After browning meat or onions, small brown deposits are left at the bottom of the pan: the fond. They have a lot of flavour. Deglazing is pouring a liquid over them to loosen them. Reducing is letting it boil to concentrate. Two minutes, and you have a sauce.
Pour the liquid into the still-hot pan and scrape the bottom with a wooden spatula. What comes loose gives the sauce its flavour.
Three minutes, while the meat rests.
Too reduced, too salty or too strong: add a little water. Too thin: let it boil one more minute. It went black: the fond burnt and the sauce will be bitter, throw it away, and next time lower the heat while the meat cooks. With wine, let it boil at least a minute so the alcohol evaporates.
The grandma-style bolognese sauce: the red wine poured over the browned meat is exactly this move.
If you cook on gas, pour the wine off the heat and put the pan back afterwards: the alcohol can catch fire.