实实在在 is a fixed four-character expression (成语) that intensifies the meaning of 'real' or 'true'. It describes something that is genuine, concrete, and not exaggerated, or a person who is honest and reliable. You can use it as an adjective before a noun (with 的) or as an adverb before a verb (with 地). It often appears in spoken and written Chinese to emphasize authenticity or down-to-earth quality.