Definition of insta-
What we mean by "insta-" is the same as "snap". In French, it could be translated as "a game made at lightning speed". We can insta-call, inta-fold, insta-shove, insta-raise, etc.. The mention of the word "insta" indicates that the game was made without thinking. It can show a lot of strength. For example, if you bet on the river and after your game, 0.1 second later, your opponent announces "all in", we'll call it a insta-all-in. Usually a player will make this game when he is strong.
Good players don't make a insta-
The problem with the games that we make "insta-" is that we give information to our opponent. For example, if a player is betting on the river and you missed your draw, you'll think about it. You will evaluate the options: can I call and win? Which hand does he have? If I raise him, will he fold? Is this a good bluffing situation? All of this is worth thinking about and time. When you do a insta-raise all in, most of the time you will be very strong. Of course, you can use the insta-all-in to make your opponent think you're strong, but it's not recommended. As a general rule, no matter what game you want to play, take a few seconds before you do it.