A booby trap was a secreted device that would attack those who triggered it.
A member of professor Parry's archaeological expedition to the planet Telos was electrocuted after trying to open the door that lead to the Cybermen's tomb. The Cybermen set the booby traps to test the intelligence of those entering their resting place. The Second Doctor helped the expedition to disarm the electrified door and to avoid other dangers inside the tomb. (TV: The Tomb of the Cybermen)
When Eldrad did come back, after one hundred and fifty million years, a door he opened triggered a booby trap. A dart containing an acid that neutralises the molecular bonds and eventually will shatter the crystal matrix in his silicon-based body hit him. As there was no antidote, the acid being of Eldrad's own design, he asked the Fourth Doctor to deliver him to a regenerator chamber.
They dodged other traps on the way, including a motion triggered gas in a passageway on the level 306 and some falling rocks.