Toxic positivity sounds like a bit of an oxymoron when you first hear the term, you stop and ask yourself how something positive can ever be toxic, but the concept doesn't attack the act of being positive in itself but rather the denial of negativity. To put it another way, in Greek legend, Daedalus crafted wings for himself and his son Icarus made from feathers and wax. Daedalus warned Icarus not to fly too close to the Sun but Icarus threw caution to the wind when he took flight and despite all warnings flew too close to the Sun, the heat melted the wax and he fell to his death.
You will have no shortage of people in your life telling you that you can't do something, criticism comes easy to most people. I've written before about how as human beings we're conditioned to focus on things that are wrong and ignore the things that are right because problems need to be solved, solutions do not. There will be times when you come across the opposite however, often described in a rather derogatory way as cheerleaders, their raison d'etre is to build you up and give you motivation and the belief that you can achieve anything you put your mind to with enough determination and hard work.