As others have said, they're character graphics. Much used in ZX81 games because the ZX81 didn't have direct support for full resolution (256 x 192) bit mapping.
There were a couple of hi-res hacks - pseudo (partial_ hi-res, which was a hack of the character system, and full bit-mapped hi-res which only worked with internal RAM or special modified external RAM.
Both were a bit of a project (for the time.) Some games used them.
My fav ZX81 program was the 1K chess. Very basic character graphics, but it somehow managed to fit a minimal but workable chess engine into 1K of Z80 machine code.
There were a couple of hi-res hacks - pseudo (partial_ hi-res, which was a hack of the character system, and full bit-mapped hi-res which only worked with internal RAM or special modified external RAM.
Both were a bit of a project (for the time.) Some games used them.
My fav ZX81 program was the 1K chess. Very basic character graphics, but it somehow managed to fit a minimal but workable chess engine into 1K of Z80 machine code.