The terms stereo and mono are often used when referring to amplifier connections. A stereo amplifier has two independent channels, one left and one right. The left and right signals of the stereo signal are similar but not exactly the same. The two channels are used to give the audio a sense of depth. If one instrument or voice is only produced in the left channel, it will seem to originate from the left side of the listening area. If a particular sound is only slightly louder in one of the channels, that sound will seem to originate off center slightly toward the channel in which the sound is louder. If you have two speakers but supply mono signal to both of them, there will be no sense of separation or depth. If a mono signal fed to both channels of a stereo amplifier, with a speaker on each channel, the output will mono. If a stereo signal is fed to the same amp/speaker set up, the output will be stereo. If a speaker is bridged onto a stereo amplifier, the output of the speaker will be a mono output, even if the signal fed into the amplifier is a stereo signal. Even if 2 speakers are bridged onto the amplifier, the output will still be mono because the output from each speaker has the same content.

Mono with one speaker:
In this diagram, the speaker is directly in front of the listening position and the audio appears to (and does) originate from the speaker.

Mono with 2 speakers:
In this diagram, you can see that the same signal is reproduced by both speakers. Since the signal content going to each speaker is precisely the same, this is a mono system. If the level of the signal is the same in both speakers, the signal will appear to originate precisely in the center of the speakers.

Below, you can see that the signal content from each speaker is the same but it is slightly louder in the right channel. This means that it will seem to originate a little to the right of center.

Stereo audio:
In this diagram, you can see that the 'x' portion of the audio is reproduced equally in both channels and appears to originate in the center of the 2 speakers. The 'y' portion of the audio is only in the left speaker and appears to originate from the left speaker's position. The 'z' portion of the audio is only reproduced by the right speaker. This means that it will appear to originate from the right speaker's position.

Below, you can see that the 'y' portion of the audio is produced in both channels but is at a reduced level in the right channel. This will cause the 'image' of the y part of the audio signal to appear to originate from left of center (not the far left or the center). This is how the audio 'stage' is reproduced with a stereo signal (different signals are recorded/reproduced at different levels in each of the speakers).