what is an alternating signal?

Answer 1

One which continously changes between two states.

For instance the a.c. mains voltage in your house is an alternating "signal." It changes its direction of flow 50 or 60 times a second, depending on where you live.

Radio waves, whether they are "radio", t.v., cell-'phone, radar or whatever, are alternating signals. The polarity of their electric and magnetic fields change at their stated frequency.