This graph uses monthly global temperature data from the atmosphere near the Earth (the lower troposphere). Some months are low and some are high, but a long-term view reveals an upward trend. This is not a plot of actual temperatures–that’s not especially useful scientifically–it’s a plot of temperature anomalies: the difference between each month’s temperature and a baseline that is essentially the average temperature over a long period leading up to the period we’re looking at. A positive number indicates the amount in degrees Celcius above the baseline, negative is below the baseline.
There are many ways to determine, plot and compare data on global temperatures so always view graphs like this with caution; to be truly useful scientifically, this graph would require a long list of information about exactly what datasets were used, what assumptions were made, limitations of the satellites used, how the trendline and baseline were determined, etc. That information, by the way, is available here, at WoodForTrees.org.