Here's why:

* Electron Carrier: NADH (Nicotinamide adenine dinucleotide) is a crucial electron carrier molecule in cellular respiration. It carries electrons from the breakdown of glucose (or other fuel molecules) to the electron transport chain.

* Energy Transfer: These electrons are at a high energy level and, when passed along the electron transport chain, release energy that is used to pump protons across the mitochondrial membrane. This creates a proton gradient that drives ATP synthesis, the primary energy currency of the cell.

* Metabolic Processes: NADH is involved in many other metabolic processes, including the synthesis of fatty acids and amino acids.

In essence, NADH acts like a "mobile battery" carrying high-energy electrons to power various cellular processes.