baseline
The baseline is the invisible line letters sit on, the one ruled across a page of exercise paper.
In more detail
Most letters stand on it. A few hang below — the tail of a g, the loop of a y — and that overhang is part of the design of the font rather than an accident of the letter.
The gap between one baseline and the next is what makes a paragraph look airy or cramped. It is set as a distance rather than as a proportion, so two fonts of the same size can be made to line up across a column.
What it does on a real document
Where a line of text lands is said by where its baseline lands, not by the top of its letters: the top moves with the font and with the letters chosen, and the baseline does not.
That is why a block of text is given a frame and a gap between lines rather than a position per line, and why the first line of a block can be asked to sit on a stated height.
Where it comes from, and why
It is a scribe's line before it is a printer's: ruled on the parchment, written on, and left visible. Type kept it as the one line every letter of a font is designed against.
Not every writing system uses it the same way — some hang their letters from a line above instead — which is one of the things a font has to declare about itself.