When used, it will give +2/+2 to all monsters in your hand, but also increase their cost by 2G. While this might seem rather underwhelming, it actually is extremely good and is a real threat if the holder uses it right. On top of that, it also only costs 2G to use.
Best used on cards with a cost of 3-4, 8-9, etc. to make them able to be affected by your passive, making them technically only cost 1 more.
Also works very well on cards with 1-2 ATK in order to prevent death by Muffet's Pet. Such cards include Ice, Dog House and Mettaton, but also cards with helpful "At the start of your turn" abilities like Memorial Statue, Gaster and Sans.
With the +2/+2 boost, monsters will also survive longer whilst dealing more damage. This can be helpful with monsters like Knight Knight and Papyrus, who can more easily kill monsters to make the best out of their abilities, while also able to survive more attacks. Surviving longer can also help monsters like Receptionist 1 activating its ability more often.
And finally, monsters with Dodge or Armor can also be much harder to kill with the boost, and will deal out more damage to monsters that do some chip damage to them to be killed. Because of this, Inflation is also very helpful on Receptionist 2 and Sad Dragon.
At the cost of 2G, Inflation, as well as the earlier revealed Acceleration, Bravery's new spell, are 2 of the 3 Epic cards that have ever been lower than 3G. (The 3rd one being an old version of Napstablook.)