A 5.1 receiver has 5 amplified speaker outputs plus 1 unamplified subwoofer output. A 7.1 has 7 plus 1, a 9.2 has 9 plus 2, etc. There is sort of a standard setup for 5.1 with Left/Center/Right and Surround Left/Right. The 7.1 adds Rear Surround Left/Right. The 9.1 adds Front Height Left/Right or Front Wide Left/Right.
However, some AVRs give you the option to "assign" the speaker outputs differently. You can take 2 of the outputs and run a pair of speakers in another room. So you could have a 7.1 AVR driving only 5.1 in your TV room, and 2 speakers in a different room. This kind of setup would require no additional amps, but you may be limited as to what you can do with it. For example, you may not be able to watch TV w/ 5.1 AND listen to a CD in the other room at the same time. Read the manual to find out.
In addition to the powered speaker outputs, some AVRs have un-amplified speaker outputs. They're called Pre-Outs as they are Pre Amplifier. They do require another amp to drive speakers. So you run the pre-outs to a separate amp, then to speakers in another room. This is the Zone 2 and Zone 3 you see. An AVR with Zone 2 will have 2 pre-outs for a pair of speakers. An AVR with Zone 2 and Zone 3 will have 2 pair of pre-outs. The advantage of Zones is that you CAN play different sources at the same time. You can be watching TV in one room and listening to music in another room at the same time. The disadvantage is the preouts need additional amplification to drive speakers.
For maximum flexibility for your grandparents with a 5.1 TV room, plus a pair of speakers outside, plus another pair of speakers inside, you would want an AVR with Zone 2 and Zone 3 outputs, AND an amp for Zone 2, AND an amp for Zone 3.
If they would only want a single source playing at any one time, like a CD in the TV room, outside, and separate room of the house, all at the same time, you could just do a 9.1 system. Put in the CD, put your AVR on All Channel Stereo, and all rooms would play the same thing at the same time. But again, it might be a bit cumbersome. Turning speakers on/off and adjusting their volume has to be done in your AVR Setup menu. It could be a little complicated and tiresome for your grandparents. Even using Zones will take a bit of learning on their part, and may not be something they like.
Have you considered a separate AVR for the separate rooms instead of running everything from one?