A suffix is an element of a language that is added to the end of a word. E.g. -ly is a suffix often found at the end of adverbs: really, quickly, happily, strangely, etc., -d/-ed is a suffix often found at the end of a verb to denote the simple past: used, bruised, grazed, heated, etc.
Several languages in which English has its roots have easily definable rules. For example, sticking "A" in from of an adjective can mean the opposite of that adjective (Asymmetrical, symmetrical), ...