Why MUF is the Maximum Usable Frequency, the highest frequency that the ionosphere will refract back to Earth for a specific path between two points at a given time. Above the MUF signals punch through the ionized layer into space instead of bending back down. It is path-specific and changes continuously with time of day, season, and solar activity, so it is not a single value for a 24-hour period. The companion term LUF, Lowest Usable Frequency, sets the bottom of the window where absorption and noise still allow a usable signal.
Watch out The choices tying the figure to a 24-hour period are wrong because MUF is quoted for a particular path at a particular moment; and Minimum Usable Frequency confuses it with the LUF at the other end of the range.
MUF = Maximum, path between two points. Work just below the MUF for the best skip; above it, signals escape.