%8.1f is a format specifier. This should point you in the right direction for figuring out what all the parts mean. It specifies the format in which a variable will be printed. In this specific case, it is a place-holder for a floating point variable. It reserves a width of at least 8 characters and a precision of 1.
For more information, do some research on format specifiers. It’s an extremely well documented and widely available topic for virtually every programming language.
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solved What does ‘%8.1fC\n’ mean? [closed]