What is Rx?

An ASCII-friendly way to spell ℞, an R with a crossed right leg, shorthand for the Latin singular imperative recipe! “take (the following)!”, which is the conventional start of a prescription; by extension, something necessitating a prescription to buy.

It's the odd North American English "abbreviation" for a "prescription medicine". Where a "prescription medicine" is one that cannot be bought directly by a consumer and instead requires a medical professional to prescribe it: https://en.wikipedia.org/wiki/Medical_prescription

It's a standin for "Prescription", but for more details check wikipedia: https://en.wikipedia.org/wiki/Medical_prescription

(used to be Latin for "Recipe", aka, here's how to make this medicine)

Prescription drugs

Prescriptions