Some of the most important quantities in physics are conserved : What they measure can neither be created nor destroyed, and their total sum is constant over time. Such statements of constancy over time are called conservation laws.
The most important conserved quantity is energy. Energy can neither be created from nothing nor simply vanish. If the energy contained in a system increases, it must be because energy has been transported into the system (and there is now less energy outside the system); if the energy decreases, it must be because energy has been transferred from the system (and there is now more energy outside).