What does the blue color section in background mean?

[lampahome]

I tried to plot graph about a time-series with library statsmodel.

I decide to plot autocorrelation function, but I don't know the blue
section in the example graph mean...

Can anyone tell me?

The plot_acf example link:
https://www.statsmodels.org/dev/generated/statsmodels.graphics.tsaplots.plot_acf.html#statsmodels.graphics.tsaplots.plot_acf


The  graph in example:
https://www.statsmodels.org/dev/plots/graphics_tsa_plot_acf.png

 thx