What does “ounce-counting” mean?

Ounce counting is outdoor-enthusiast jargon for minimising the weight of equipment. Heavy equipment wears out its wearer, so some backpackers, hunters, mountaineers, etc. pay a great deal of attention to how much external weight they carry. They are referred to as ounce-counting backpackers, hunters, mountaineers, etc.

Over the years I have touched on ounce counting when backpack hunting and how I’ve developed a system to help me drop significant pounds (not just ounces, but pounds) from my “going in” pack weight.

— Eastmans' Issue 156 - OCD (Ounce Counting Document) Packing

Marketers use the term as an adjective to attract their target audience of serious adventurers:

If you are the type of ounce-counting backpacker who hasn't been able to justify carrying a pillow in your pack the ergonomic Exped Air Pillow UL may change your mind.

— Exped AirPillow UL (2020)