The ZX81 is not renowned for the quality of its graphics, which, to say the least, are chunky, but this program is designed to give you better definition graphics. It is essential that you read the enclosed booklet, in itself disappointing with microscopic print, as you would certainly be confused if you just ran the program hoping to pick it up from the screen.
However, once you start entering the example programs provided, things will start to become clearer. But I still don't understand wht the author shows a byte with the least significant bit (0) as the leftmost bit, while giving it the value 128, the most significant bit of the byte. Nevertheless the program does work, and it is worthwhile persisting until you get the hang of it. Of particular interest to many will be the ability to create quite large sprites, which, with a curious mixture of pixels and characters can be a maximum of 40 x 64 pixels.
A bit overpriced, maybe, but a valuable tool for ZX81 programmers, in either Basic or machine code.