Before iPhones took over the world, there was the good old classic phone. Nokia was a world famous brand that dominated the market, and, because mobile phones were just starting out, they were just mobile, not "smart".
Their main purpose was calling (and receiving calls) - so their keypad was optimized for that, containing just the numbers and the most common symbols. Over time, writing text started to become a thing, and so, each number was mapped to different letters from the alphabet.
By pressing it consecutively, you could cycle through the letters, and by pressing it for a long time, you could add the number itself to the message. This was what was used to send SMS in the early days.
Because SMS stands for Short Message Service, the message was limited to 160 characters.