RTTY uses the Baudot code, invented before radio even existed, and still widely used throughout the world. The Baudot code uses data bits to represent letters, numbers and punctuation, much like your computer does. Unlike your computer, which uses eight bits for each character, the Baudot code uses only five, plus a start bit and stop bit. Using fewer bits is good because it speeds up transmission and reduces the chance of errors, but there is a complication. Five data bits can only represent 32 different characters. Since there are 26 letters in the English alphabet plus ten numbers, plus some punctuation, 32 different characters is not enough, even if you only use capital letters, which Baudot does.
Mr. Baudot could have chosen to use six data bits or even more, but he found a better solution. He reasoned that most of what would be sent would be letters rather than numbers or punctuation, so he assigned all the letters to the basic 32. He then had six characters left over and he did a very clever thing with two of them. He made one of them a FIGURES SHIFT and another a LETTERS SHIFT. The way it works is this: When sending one of the basic 32 characters, nothing special happens. But when a number or punctuation is to be sent, a FIGURES SHIFT character is sent first (it's a non-printing character - you won't see it on your screen).
Whatever follows will still be one of the basic 32 characters, but the receiver will interpret it differently. For example the letter Q uses the same five data bits as the number 1, but when the receiver gets a FIGURES SHIFT first, it prints the next character as a 1, not a Q. This continues until a LETTERS SHIFT character is received, at which time the receiver goes back to "normal" printing. All of this shifting is done by the system - there is no key marked LETTERS SHIFT or FIGURES SHIFT. It's all automatic and you will scarcely notice it happening.
In fact, the only reason to mention it at all is because we are using radio instead of wires, and radio is susceptible to interference from various sources such as lightning static, man-made noise, etc.
If a burst of static should happen to wipe out a LETTERS SHIFT or FIGURES SHIFT character, the characters following will not print correctly until another LETTERS SHIFT or FIGURES SHIFT is received. For example, suppose you are sending a signal report of 599, but a burst of static wipes out the FIGURES SHIFT character. Instead of printing 599, the other fellow's computer will print TOO. TOO is exactly the same as 599, without the FIGURES SHIFT. We all got used to interpolating "shift" transmitted reports and serial numbers in the early days!