A multimeter measures voltage, current and resistance with one instrument and a pair of probes. Most also test continuity (with a beep), check diodes, and some measure capacitance or frequency. It is the first instrument for any fault: is the supply there, is the fuse good, is the cable connected end to end.
Connecting it
Ammeter: break the circuit and put it in the path. Voltmeter: touch it across the part.
A voltmeter has very high resistance so it barely disturbs the circuit: touch it across the part. An ammeter has very low resistance so it can sit in the current's path: you must break the circuit and put the meter in the gap. The small voltage it drops there is its burden, usually unnoticeable but worth knowing for sensitive circuits. For heavy currents (the several amps a transceiver draws) use a clamp meter, or a meter with a high-current range, within its rating and for short times.
Left: the ohmmeter makes its own test current. Right: a charging capacitor reads ever-higher resistance.
An ohmmeter makes its own small test current, so the circuit must be unpowered, and the part ideally isolated, or other paths in the circuit will give a false reading. Discharge capacitors first.
Loading: the meter is a resistor too
Source resistance and meter resistance divide the voltage between them. The meter reads only its share.
Every voltmeter draws a little current through its own input resistance, which forms a voltage divider with the circuit's own resistance. A 10 MΩ digital meter on a 1 MΩ source reads 10 ÷ 11 = 90.9% of the true value; on a 100 kΩ source, 99%. An analog meter is much worse: a 20 kΩ-per-volt meter on its 10 V range is only 200 kΩ, and it reads 66.7% on a 100 kΩ source. Remedies: measure where the impedance is low, use a high-impedance meter, or know the error.
Reading AC and RF
Most meters read AC volts correctly only for a sine wave at the mains' frequency, or a little higher. An ordinary meter measures the average and scales it to RMS, so a distorted wave reads wrong; a true-RMS meter handles other shapes. Meters cannot read RF directly: their circuits stop at roughly the audio range. A diode RF probe converts RF to a DC level for a DC meter, giving a rough indication, not a calibrated measurement. Use a scope or a power meter for RF.