Sondell spelled it out correctly.
You use the Target to size your final output file, as VBR will be used. AME will then try to average as close as possible to "average out" to your target, yet the actual bitrate could be much lower, or up to your Maximum Setting.
The Maximum Setting is in place because of bottlenecks which exist for playback; for instance, many Blu-Ray players cannot read anything higher than 30 Mbps. DVD Caps around 9 mbps I believe.
The one part that I would like to point out is I recommend always using 2-Pass VBR, never one pass; unless your content is very consistent frame for frame.
AME/PPro lack a "Minimum BitRate"; which is a shame. Using VBR 1 Pass can cause pixelation issues; such as when a camera flash goes off - blowing out to near all white a single discreet frame. As a 1 Pass encode does not pre-analyze the program stream prior to setting the bitrate for a given frame set; a one pass VBR can cause artifacting after a camera flash; or a white or black hard edit to picture.
Also, any flickering type of video as well, such as strobe lights, can also cause this. Basically; if your image goes from flat color (any color) to complex picture/moving image in a 1-2 frame span... there can be trouble.
This occurs because it sees the flash as a blown out white image... thus AME drops the bitrate down very low (think under 0.5 Mbps); because after all; its just a frame of white... high bit rates are not needed.
The issue is that AME, its scheme for how it ramps up and down the bit rate is not fast enough; when using 1 Pass VBR. So after a camera flash goes off; your next frame, or even next two frames; could appear blocky, as the VBR bitrate has not ramped back up fast enough, and the flash lasted for only a single discreet frame.
2 Pass; while it takes longer; does a much better job handling this.
To be perfectly honest; I master nearly all of my content using CBR. While this goes against the mainstream norm; I know what bitrates are needed for what quality I desire; I'm not happy with the 1 Pass Encoder Issues; and 2 Pass is a time consuming render.
There are cases where 2 Pass is necessary; such as long form online content delivery, but for most outputs CBR is fast, clean, and consistent.