The ZX80 and ZX81, like a number of 80s personal computers featured a set of graphical characters in addition to standard alphanumeric and punctuation and provided a way to enter then directly from the keyboard. On the Sinclair machines you'd use SHIFT + GRAPH to enter "graphics mode."
In addition to other answers, ZX81 BASIC had a PLOT command to draw a "dot" on the screen. However since the display was character based (32x24 grid, but the bottom two lines are reserved) what this command actually did was place one of 16 2x2 black and white blocks at the particular character location (giving 64x44 resolution). If there was already a non-block character at the location it was replaced. (Also UNPLOT which was the same except drawing a white quarter-pixel). There were no line or circle drawing commands, so you had to implement that yourself.
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.
