Avocado toast is a food item, usually used for lunch, breakfast, or a light snack, consisting of avocado spread across toast. This is a ridiculously easy food item to make, the only considerations being finding an avocado that is ripe enough to spread easily, and finding the right type of bread to hold the avocado. Also, of course, there are different ways to garnish the two basic ingredients, adding dashes of different spices and sauces. My own standard way to prepare it is to melt a little bit of mozzarella cheese on top of a marraqueta and then to add the avocado.
That being said, the seemingly humble avocado toast has become something of a cultural phenomena in the past year, being an example of the type of thing that flighty millenials are posting on their instagrams. At least one think piece has been written describing avocado toast as a symbol of millenial profligacy and avoidance of adult responsibility. And indeed, in certain places, avocado toast is a 10 dollar menu item that seems to serve purposes of being trendy rather than culinary taste. (I had to explain to some of my Chilean students why a news article referenced "avocado toast" as a symbol of frivolous eating, when in Chile, avocado toast is the exact equivalent of a peanut butter sandwich in the United States).
That being said, while avocado toast might be expensive, do you know what is a lot more expensive? Heart attack, stroke, and other cardiovascular disease. Things that can be prevented by eating fresh vegetables. The math here lines up well: the current price of avocados is 1 dollar per unit. An average two day hospital stay for a mild heart attack costs 10,000 dollars. There are 10,000 days between the time you are 20 and the time you are 50: so if you eat avocado toast every day from the time you are 20 to the time you are 50, and avoid that heart attack, your avocado consumption has just paid for itself. Obviously, these numbers are very round, and also obviously, you can't magically prevent all cardiovascular problems by eating a single avocado a day, but in general, while healthy eating might seem expensive, it is a lot cheaper than the alternative. And, in the case of the smooth, creamy wonder that is avocado toast, much more pleasant.