If you're assured, you act with self-confidence and certainty. An assured candidate is more likely to get a job than a hesitant, doubtful one.
An assured statement is said with conviction, and an assured football team has an advantage over an uncertain one, simply because of their confidence. It can be uncomfortable to be the audience for a nervous performer, but an assured one is a pleasure to watch. The adjective assured comes from the verb assure, with its Old French root, asseurer, "to reassure, calm, or protect."