AM - Ante Meridian, in Latin (Before Noon, in English)
AM is derived from Latin which represents time after noon and before midnight. "Ante" means Before and "Meridian" means Middle. So, this is the time of day before the sun has crossed an imaginary North-South line passing directly in the middle of the sky.
It is used to represent time, from midnight to just before noon. Like in a sentence we can say - "I will reach your home at 09:00 AM". Here the time is 9 o'clock in the morning.
AM - Amplitude Modulation
Amplitude modulation is a technique in which, strength of the transmitted signal is varied, for transmitting information via a radio carrier wave.