What is meant by the terms 'active high input' and 'active low input'?

The term "active low" means that the input on an IC requires a logic low for it to be activated, i.e a low voltage (of course the voltage range is dependent on the technology, TTL, CMOS etc) Active high is the opposite... the input requires a logic high for it to be activated. A simple example to illustrate, an 8bit counter can count UP/DOWN this functionality is controlled with only one pin, an active high on that pin to count up or low to count down.