On a multimeter, the black lead is generally neutral/ground; red is hot/positive, depending on whether it is being used for ac or dc.
A typical multimeter has a very high input impedance - i.e., it will complete a curcuit, but the impedance or ability to pass current is VERY low, so if you then put a real load on the circuit, where there was high voltage before, now is zero - this is very common, and needs to be taken into consideration when electronic (your dimmer) devices are in the circuit along with the multimeter. IOW, the dimmer may have 120vac on it's hot output when 'off', but it's incapable of providing any current. A light bulb may be in the 10-ohm range depending on it's voltage/wattage rating. Through a multimeter, it may be in the 100,000,000-ohm range or higher, partly so it doesn't unduely load the circuit you are trying to measure, but since it looks sort of like an open circuit, it can 'look' like there's decent voltage/current there when there isn't.
This is where understanding the tool can be confusing and it can lead to false diagnosis.