Mode 7 was the SNES graphics mode that allowed for things like rotating and zooming a background layer - used for things like the track in Mario Kart and F-Zero, or the worldmap in Final Fantasy 6 (while you're in the airship). The Scroll Register was used for scrolling in Mode 7.
H-Blank is the horizontal blanking period (and associated interrupt) - a time period between the drawing of scanlines on the screen (there's also V-blank, between frames). Changes made during H-Blank could make for some interesting effects - it was used for things like the circle that closes around Mario at the end of a level of Super Mario World: The rectangle draw routine is used, but the size of the rectangle is changed between scanlines, creating a circle. I'd imagine this was used for some of the wavy distortion effects in games like Chrono Trigger and Earthbound as well.