Ammeters are used to measure current flow, as such they need to be inserted in series with the circuit under test so that the current to be measured flows through the meter. This means the circuit must be broken.
To connect an ammeter, the power must be switched off. The circuit is broken at the point where the current is to be measured. The meter is then inserted into the circuit in such a way that, 'conventional current' flows into the red lead and out of the black lead. Once the meter is connected, circuit power is restored and the measurement taken.
To disconnect the meter, the circuit power must again be switched off. Once the meter is removed from the circuit, the circuit must be reconnected.