In the context of electronic audio equipment, a stage is a functional block of circuitry that performs a specific task. For example, a 'four-stage' phaser effects pedal employs four all-pass filter circuits.
An individual stage could be a preamplifier, an insert return buffer, a section of an equaliser, or a mix-bus amplifier, for example, and the process of optimising the signal level passing through each stage is called gain-staging.