Definition of insta-
What is meant by "insta-" is the same as "snap". In French, it could be translated by "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 play, 0.1 seconds later, your opponent calls "all in", it is called an insta-all-in. Generally, a player will make this play when he is strong.
Good players don't play insta-
The problem with "insta" plays is that you are giving information to your opponent. For example, if a player bets on the river and you miss your draw, you're going to think. You're going to weigh the options: Can I call and win? What hand does he have? If I raise him, will he fold? Is it a good situation to bluff? This is all worth thinking about and taking time. When you insta-raise all in, most of the time you will be very strong. Of course, you can use insta-all-in to make your opponent think you are strong, but it is not recommended. As a general rule, no matter what game you want to play, take a few seconds before you do it.