what are they? i always thought its kind of physical box of some gadgets or other stuff
A few years back "microtransactions" became a popular way to fund games that might not have got by on traditional sell game+expansions, donations or straight up paid play.
What they did varied a bit between games but commonly it was seen to offer selections in multiplayer games (if you have a 30 character roster you pick 3 randomly, unless they pay in which case their selection of choice), extra skins and such like. Other games offered different weapons (this got split between pay to win where those that paid got better stuff, others paid to have a similarly potent weapon or indeed a fairly different way of playing), or enhanced modes of play (for example more draw distance in one of the turn based city building things I played once, others might offer experience boosters).
So far so shady, or perhaps just a new way to monetise. There were instances of games, often single player puzzle games offering continues a la the arcades of old, connected to apple and android stuff (possibly also MS and Sony console stuff but that got less attention) which in turn were connected to credit/debit cards and kids creating a nice bill for their parents.
At similar times it was noted that "progression" in your multiplayer game was a thing some enjoyed. Often this was governed by you "levelling up" or spending some resource gained by play and unlocking a pack with some random weapons/weapon addons/skins in it.
Not something I terribly enjoy, certainly never thought Quake or Unreal needed such a thing, but OK.
Some then figured out the combination of the two concepts would make a lot of money. They were right.
Some games also allow trading these items with friends. If you can trade it then you can sell it. This then led to the rise of sites wherein one could wager said skins on the outcomes of events which caused some fun as it then becomes a de facto currency of sorts. This is not the gambling some refer to when discussing such this and instead they refer to the more basic concept, though such things certainly got the legal types to take note as indeed appears to be the case for this.
If this sounds a bit like booster packs of old collectable card games then you are right, those also appeared in computer form by the way and displayed some of the worst excesses here, and are what some consider the gambling aspect.
Equally while it started out as smaller companies seeking a way to fund things then today it is used by the biggest game devs doing the biggest games. Star Wars battlefront 2 (a game made by EA and with star wars on the front) being one that got a lot of press for it last year, and they are far from the only ones.