Unrealized gain is the difference between a holding's current market value and its cost basis, measured while you still own it. It is often called a paper gain, because nothing has been exchanged — the position is simply worth more than it cost at this moment. When current value is below basis, the same figure is an unrealized loss.
Unrealized versus realized
Selling converts an unrealized gain into a realized one at the price the sale actually executed. Until then the figure is a mark against a market price that moves every trading day, and the number you see is a snapshot rather than an outcome. A large unrealized gain can shrink or disappear entirely without a single transaction on your part.
The tax difference
In a taxable account, realizing a gain is generally what creates a taxable event; an unrealized gain generally is not taxed while the position is simply held. How the realized gain is then taxed depends on how long the holding was owned and on rules that vary by country and account type. Tax-advantaged retirement accounts follow different rules entirely. This is a description of the mechanism, not tax guidance — the specifics belong to a tax professional and to your own jurisdiction.
Reading the number
An unrealized gain says what a position is worth relative to what it cost, and nothing about whether that is a good or bad outcome. Two portfolios showing the same total gain can differ completely in how concentrated it is, how long it took to accumulate, and how much of it a single holding produced.
NetWorthy computes unrealized gain per holding as market value minus the cost basis you entered, and totals it across the portfolio. It is a display figure derived from those two inputs — the market value moves with quoted prices, so the total changes without anything in your account changing.
See also Cost Basis.