Welcome the Forums DD. I am a fan of the engineering forums site - and although the posts there can be a bit dense, they are worth the time invested to understand a concept. In this case a post from
ENG-TIPS circa '04 is apropos while the quote it contains sums it up nicely:
"Breaking it down even further, it is: Ratio of the amount of air getting into the engine’s cylinders vs the amount that will fit. Basically, how efficient is an engine at filling its cylinders, that’s all."
