Unrealized return (or unrealized gain/loss) is the difference between an investment's current market value and the price you paid for it, for as long as you keep holding it in your portfolio.
Why it's not "real money" yet
Since the asset hasn't been sold, that gain or loss can disappear, shrink, or grow before you decide to sell. It only becomes a realized return — and a taxable event under IRPF — at the moment of sale.
Psychological effect
Watching a large unrealized gain on screen can lead to hasty decisions (selling out of fear of losing it, or not selling out of greed while hoping for more upside). Remembering that it's just a number until you sell helps you make more rational investment decisions.