These are the mini upgrades with an item slot shape and a heavy plus sign (+) in front of it on your character upgrade tree. Just like the pic below!
To access it, simply select your desired character and tap the button "Stats". Then, on the left bar menu, tap on "Upgrades". This will take you to your character's upgrade tree!
These slots will be available to all characters as mandatory mini upgrades on levels 3, 6, 9, and 14. You will just have to reach them by collecting character tokens and purchasing the mini upgrades with coins! :)
Oh, and the first one for each character will always be for free! Have fun, Zoobster!