As long as the projector indicates that it is HD compatible, the quality will be somewhere in between a true 720p display and an SD picture, but closer to HD than SD.
HD is High Definition while SD is standard definition.
1280x720 is HD video resolution.
HD or 1280 x 720 pixels originated from high-definition television (HDTV) where it originally used 50 or 60 frames per second. 1280 x 720 is referring to 1280 pixels horizontally by 720 pixels vertically. The total picture or total number of pixels produced by 1280 x 720 is 921,600 total pixels. A single pixel is simply the smallest element in display device, an element in a dot matrix. A pixel is also considered a picture element. The pixels added up produce your video resolution for the picture.
A projector is considered a display device, because it can "display" or show the picture as a projected image onto your external projection screen. In comparison, a flat panel display has a built-in screen to show the image. Both a projector and a flat panel display are considered output devices. Output devices are where you plug your source device (i.e. a laptop, Blu ray, Media player, Game console) into as your source device determines what originating signal (i.e. movie content video signal, video game content video signal, etc.) will be sent to the output device.
HD has a higher pixel count than SD. High pixel counts equate to more detail in the picture.
SD is video resolutions lower than 720p.