the Hipo engine means "high performance". It was used on the 289's in the 60's to denote the high performance package. Just sort of a short term like saying "GT" instead of grand touring or whatever.
Hi-po was short for High Performance. The Hi-po has a different block with heavier 2 bolt main caps, better rods, a slightly better crank ( really just aslightly better casting than the std ones) heads were only slightly different, they had cast in spring pockets, screw in studs w/guide plates, same ports and valves as the std heads. The exhaust manifolds were better flowing, the intake was the same as the std one, the carb was a 600 cfm instead of a 480 cfm Autolite. The cam was also different, it was a solid lifter design with improved lift and duration over the std cam. I think that was about it, I don't think I missed anything, if I did , I'm sure someone will add to it.
A few other small things unique to this car were a larger diameter "underdrive" pulley on the alternator and a steel hub/4 blade aluminum fan. The early HiPo's (65/66) were the only Mustangs of this vintage that used a factory installed 9" rear.