An unrealised gain exists only on screen: the difference between what a holding is worth today and what the shares you still own cost. It moves every time the price moves. A realised gain is locked in when you sell. The sale is a disposal, and for tax that's the moment that counts.
Outside an ISA or pension, gains realised in a tax year above the annual exempt amount are subject to Capital Gains Tax, while unrealised gains aren't taxed at all until you sell. The app works out realised gains using average cost, which is close to how HMRC pools shares, but HMRC also matches shares you buy back on the same day or within 30 days against the sale first, so the app's figure isn't a tax calculation. Rates and allowances change, so check HMRC's guidance.
The same gain is a number on one side and a tax event on the other.
Educational content only. It is not personal financial advice.