What does the error "function-definition is not allowed here before '{' token" mean when compiling the void loop() function for Arduino?

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This error message indicates that there is a mistake in the syntax of the code for the void loop() function in the Arduino sketch. Specifically, there is likely an extra piece of code or function definition that has been inserted before the opening curly brace ({) that marks the beginning of the loop function. The compiler is unable to interpret this code and has flagged it as an error. To fix the problem, the extraneous code must be removed or moved to a different location in the sketch where it will not interfere with the loop function.