false paths is the path that you don't care timing. for example, when IC finished reset, the chip go to some mode, and the mode signal will not change during working session. then, the mode signal to all other signal paths is false path.
multicycle path is the paths that the timing can be more than just one clock period. for example, there is some signals that will change every 2 clock cycles. then the paths between this signals may be multicycle paths with 2 cycles.