At the risk of the blind leading the blind, here; I will give you what I understand to be true.
You are correct that 5.1 "Surround Sound" means there are 5 speakers (LF, CTR, RF, RR, LR) and a subwoofer. 2.1 (Left, Right, Subwoofer) 7.1 (LF, LS, LR, Center, RF, RS, RR - subwoofer) 4.1 (LF, LR, RF, RR, and subwoofer)
Surround Sound amplifiers use those speakers to provide "content" as dictated by the incoming signals. If the incoming signal is "analog stereo", the amplifier's software will mix those two channels and play the content on the 5 speakers. A frequency filter will route bass signals from both channels (mixing them) to the woofer. The "center channel" will get a mix of left and right.
When you select "theater," the signals to the RF will go with no delay and the one to the RR can be delayed anywhere from a fraction of a second to several seconds to give the effect of a large room. The source is still analog stereo and the output will sound very good coming from 5 speakers.
TRUE (or sometimes called discrete) "Surround Sound" comes in several "flavors" (programs). The most popular is Dolby I, Dolby Pro-Logic, Dolby Pro-Logic II, Six-Axis, but Dolby Pro-Logic is by far the most popular. These are proprietary digital signal processes that "ride" on the analog wave form of the stereo signal and is "decoded" by the software in the amplifier.
EACH speaker gets its own signal channel, unique from the others. You can sit in a room with true surround sound and follow a jet plane on TV flying around your head from the noise as it moves from speaker to speaker.
Difference Between Stereo and Surround Sound
Stereo vs. surround for music
There is also "fake" surround sound using a process called VSS or Virtual Surround Sound. Bose uses their own proprietary system called TRUSPACE to manipulate analog stereo to "fill a room."
To get "true" Dolby Pro-Logic surround sound you need a digital signal (COAX or Toslink - optical). HDMI is also a digital signal and it will be preferred over other methods to display video on the TV and audio through the TV's speakers.