What is this type of plot and how to interpret it? I was presented to this plot and i've seen it before, however i would like to know what does the countour around the main line mean.

Ps: i couldn't find enough information about it to investigate by myself, so i had to create this question.
The shaded area likely shows the dark green line plus or minus some error/uncertainty estimate. Common error estimates may be based on the standard deviation, a confidence interval, or the interquartile range depending on the data and the analysis being done. Without more information, we cannot know what the shaded area represents.