Please watch the below YouTube video for complete explanation.
https://www.youtube.com/watch?v=mf4ABpHqgag&list=PL2-GO-f-XvjBl5fpzdfYaPW28PwsLzLc4&index=26
Please find below the SQL Query -
Select Category, Year, Sales from
( Select Category, [2015], [2016], [2017], [2018], [2019], [2020]
from dbo.Sales ) OrigSales
UNPIVOT
(Sales FOR YEAR IN ([2015], [2016], [2017], [2018], [2019], [2020])
) as UNPIVT_SALES