If I buy a share of a company for $1 January 1 2019 and it is worth $2 January 1 2020 I've got $1 of 'unrealized gains'. I pay zero taxes.

If I sold it before 1 year of holding, I'd pay $1 in taxes (the gain) at my top tax rate based on my other income. If I sell it after 1 year I pay tax at a reduced rate on the total gain.