An ammeter is the meter that measures the electric current in a component. Current is measured in amperes, or amps (A). The ammeter tells you how much charge passes through the component each second.
To measure the current in a component, the ammeter must be connected in series with it. This means it goes in the same loop, in line with the component, so that all of the current in the component also passes through the ammeter. You break the circuit next to the component and put the ammeter into the gap.
An ammeter has a very low resistance, so it hardly changes the current it is measuring. If you connected it in parallel across a component, it would give the charge an easy path and a very large current would flow. A voltmeter is different: it is connected in parallel with a component, because it measures the potential difference across it.
