Pictures made of letters
ASCII art draws an image with characters: dense ones like @ and # for dark areas, light ones like dots and colons for bright areas. The tool divides your picture into cells, measures the brightness of each and picks the matching character. Character cells are about twice as tall as they are wide, so the number of lines is halved to keep the proportions right.
The darkest and lightest tones of your picture are stretched to the full range of characters, so even a low contrast photo gives a readable result.
Width, characters and inverting
Width in characters sets the detail: 60 to 80 fits a chat message or a terminal, 120 and more shows faces and fine shapes. Standard uses ten classic characters, Detailed about seventy for smoother shading, and Blocks uses shaded block characters for a solid look.
Light text on dark flips the brightness mapping for dark backgrounds such as terminals and dark mode editors; the preview and the PNG switch to a dark background too. Copy text puts the art on the clipboard; paste it where a monospaced font is used or the lines will not line up.