Campfire is Kindness' 1st Deltarune, 3rd Common (4th if including Force of Nature) and 9th overall Spell added in Beta 43.0. Previous Versions can be found here.
Campfire first effect works as a healing version of Undyne('s Spears), healing your monsters 1 HP at a time. It will restore up to 4 HP, but can target monsters already at fully HP, wasting it.
It also has a 2nd effect, which works much like Space Cooler, giving +1/+1 to a number of monsters in your hand (albeit only once and right away, of course). In this case, it targets 3 monsters.
As a result, at full power, this spell will result in a total of 4 HP restored and +3/+3 for just 3G, making it very useful and powerful. However, to guarantee this, the only way is to have 3+ monsters in your hand and only 1 monster on your board with its current HP at 4 or more below its max (or more than 1, but all need to be at 4+ below their max).