This is a really good question and it is not always clear in articles when they mention different amounts of warming. Typically, when articles say this, they refer to warming since the industrial period. The IPCC quantify amounts of warming relative to the period between 1850-1900 as this is when we saw the greatest change in greenhouse gasses and subsequent climate changes resulting from human activity.
So, warming of 1oC, is 1oC warmer than average temperatures observed before the industrial period.
Hope this helps