I'll give you one specific example that I know of first hand:
The game Overwatch has paid lootboxes. These lootboxes contain only cosmetic items, sprays or voicelines. You get one for free every time you level your profile up one level, they have 4 items in each box that are all cosmetic and there are different rarities for certain items making them less likely to appear in lootboxes.
You can also spend real world money to purchase these same lootboxes and open them and get those random items. They are a form of microtransaction, but because you aren't guranteed any particular item they are considered gambling by many people. The idea being that you are paying real money without even being able to ensure you get what you want. You are essentially being forced to gamble to aquire certain items in the game if you really want them.
The argument is that a certain percentage of humans will have addictive personalities or are too young or immature to gamble within their means and its predatory towards those types of people. That's not a big deal since its been the argument against gambling forever.
I personally don't have an issue with the way overwatch handles this type of thing becaue they give you no competitive advantage, can be aquired for free if you put in the time and allow the game developers a secondary revenue source so they can continue to add new content like new characters, modes and maps to the game all of which is always free.
Only the lootboxes cost more money once you've bought the base game.
Most games that use a form of lootbox or gambling do not do it this way. Its usually random items that DO effect gameplay and give you a competitive advantage since that's how most free to play games function and make money at all.
Non-gambling style microtransactions would be ones where the price of a specific item is clearly stated and you know exactly what you get for that money. An example of this would someone buying a particular character skin in that fortnite game for their character. It might cost about 10 dollars, but you know exactly what you get for that ten dollars. There's no gambling or luck involved.
Hope that helps.