'font.sans-serif':['simhei','Arial']

This configuration sets the default sans-serif font for a document to "simhei" if it is available on the system, otherwise it falls back to the "Arial" font.

The "font.sans-serif" property is a part of the matplotlib library in Python, and it allows users to customize the font family for their plots and figures. By specifying a list of font names, the library will try to use the first available font in the list, and if none of them are available, it will use a default font.

In this case, "simhei" is a Chinese font that supports Chinese characters, and "Arial" is a common sans-serif font that is widely available on most systems. By setting both fonts in the list, the configuration ensures that the plot will be rendered with a Chinese font if possible, but will still look legible if "simhei" is not available.