It is an assembly of light emitting diodes which can be powered individually.
They most commonly emit red light.
They are arranged and labelled as shown in the diagram.

Powering all the segments will display the number 8.
Powering a,b,c d and g will display the number 3.
Numbers 0 to 9 can be displayed.
The d.p represents a decimal point.

The one shown is a common anode display since all anodes are joined together and go to the positive supply.
The cathodes are connected individually to zero volts.
Resistors must be placed in series with each diode to limit the current through each diode to a safe value.

Early wrist watches used this type of display but they used so much current that the display was normally switched off. To see the time you had to push a button.

Common cathode displays where all the cathodes are joined are also available.

Liquid crystal displays do a similar job and consume much less power.

Alphanumeric displays are available which can show letters as well as numbers.