The most common example of an infusion you are likely to encounter in your daily life is your morning cup of coffee. An infusion is the creation of a new substance by steeping another substance in a liquid, usually water.
Infusion is often misused with transfusion, which has a slightly different meaning. A transfusion, as in a blood transfusion, is the simple addition of one substance to another, without the creation of something new. But a drip feed of medicine, for example, is technically an infusion as it alters the state of the blood. See, you’re halfway to becoming a doctor (or at least a linguist) simply by reading this site.