One BTU is about 1.055 kilojoules, roughly the heat released by burning a single matchstick. On its own it is tiny, so refrigeration and air conditioning capacity is quoted as a rate, in BTU per hour. One kilowatt of cooling is about 3,412 BTU/h, and one ton of refrigeration is 12,000 BTU/h.
You will see BTU/h on compressor datasheets, condensing-unit catalogues and air conditioner labels, while many European and Indian technical documents use watts or kilowatts instead. Both describe the same thing: how quickly the system can remove heat. Converting between them is simple arithmetic, and it is worth doing when comparing a quotation in one unit with a datasheet in another.
Remember that a capacity figure is only meaningful with its conditions. A compressor rated at a certain BTU/h at a mild evaporating and condensing temperature will deliver noticeably less in a freezer running at −25 °C beside a 42 °C kitchen wall. Ask what conditions a rating refers to before comparing two systems.