Seven-segment display is a basic hardware component to output a numeral and, with some limitations, a text. They are very simple and power-efficient, that makes them still popular among manufacturers.
There are seven segments (hence the name) and additional “dot” indicator for decimal point. Most modern displays utilize LEDs, while there are models with electro-mechanical segments, or based on vacuum fluorescent tubes and so on
Main usage
As cheap LCD and OLED displays gradually replace 7-segment displays, the last ones are typically used in relatively simple devices, where there is a need to output numeric value, like simple mass-market consumer electronics (clock, ovens, etc.).
The more interesting use case is to output the value near the knob, especially if it is a rotary encoder, to provide feedback. It is typical with the overhead panel in the airplane cockpit.
Text display
As it was mentioned, 7-segment displays may be used for limited text output. Because the amount of segments is limited, it is basically not a real latin alphabet, but some kind of its imitation, but it gives the opportunity to output basic text messages, like errors. If there are several displays in a row, by using scrolling text it is possible to output relatively long messages with limited set of displays.