"Linepack" refers to the volume of gas that can be "stored" in a gas pipeline. Thinking back to basic chemistry, gas can be compressed (unlike liquids). Think of the classic example of compressing air in a bicycle pump. The air can be compressed into a smaller volume, or more air could also be squeezed into a fixed volume - for example a tyre.
The operational implications of line pack mean that the volume of gas injected into a pipeline (at the inlet), can bd greater than the volume of gas withdrawn from the pipeline (at the outlet). This frequently occurs due to the unpredictable nature of end-user operations and hence, their gas demand.
However, when gas is 'stored' in the pipeline by compressing it, the pressure exerted on all parts of the pipeline increases. The quantity of additional gas volume that can be stored in a pipeline depends on the pressure rating of the pipe, flanges, non return valves, compressors etc, as well as the ability for equipment upstream (before the inlet) and downstream (past the outlet) to respond to a sudden surge in pressure if inlet or outlet valves failure. This event is called a high-pressure/low-pressure breakthrough. When the pipeline pressure is high, it becomes increasingly difficult to inject additional gas into it. Compressors are used to increase the injection pressure of the gas.