ambient temperatures are the temperature of the air within the enclosure.
for me: i have a digital thermometer that i place in the middle of the enclosure and use that to gauge my ambient temperatures. i (as well as my snakes) move them around sometimes so i can get a better idea of the overall temp.
depending on how you heat the enclosure can change how you should be measuring your ambient temps. if you're using a single lamp (either bulb or CHE (ceramic heat emitter (i prefer CHEs))), you measure the ambient temp directly under the lamp and adjust the thermostat/rheostat as needed. with two lamps you need to be measuring both using the same method as with one. for a RHP (radiant heat panel) a thermostat probe will measure the air in the enclosure and the thermostat will regulate the power as needed.
did this help? lmk if you have more questions!