Jalapenos are hot red peppers that require a hot climate to grow on earth. As you might expect, Jalapenos in Forager are also found in hot regions. Once you unlock an area of the map that contains desert, Jalapenos will begin to grow naturally without you needing to do anything at all.
Jalapenos can be eaten raw much like berries can. They are pretty much the desert equivalent of berries. Eating them raw will not burn the mouth off you so collecting them whenever you find them is a good idea.