Digestion is the process your body goes through to break the food you eat into substances that it can absorb and use. Chewing is an important first step in digestion, because your teeth make food small enough to swallow.
Your digestion isn't usually something you spend much time thinking about, unless you eat something that upsets your digestion — then the process can be uncomfortable. You can also describe the breakdown of other types of material — when it's exposed to chemicals or bacteria, for example — as digestion. A figurative kind of digestion happens when you think through complicated information and begin to make sense of it all.