An ammeter measures electric current (in amperes) flowing through a circuit:

It can measure both direct current (DC) and alternating current (AC)
Analog ammeters use a coil and needle that moves in proportion to the current
Digital ammeters use an analog-to-digital converter (ADC) to show the reading on a screen
It has very low resistance, so it does not reduce the current it measures
Always connected in series with the component, so the full current flows through it.