A mixer is in principle a multiplier; it multiplies for instance the local-oscillator signal with an IF input signal. Then pairs of multiples are generated in frequency spectrum. We typically choose one of them, name it RF and use filters to suppress all others. Such action is named "upconversion" and can be used to modulate a low-frequency (audio, video) on a radio-frequency LO signal to get RF output. Or you can take an already modulated RF signal say at 1...10 MHz, and upconvert it to >100 MHz by the same process.
In receivers, the process is reversed, a RF signal is multiplied by LO signal, and low-frequency beat is generated as desired.