Think of a line laid down first, before anything else happens. Nothing is built, drawn or judged until that line is there. Once it is, everything else takes its position from it: you measure out from it, you line things up on it, you decide what is inside it and what is outside it. The line itself is nothing special; its whole value is that it came first and it does not move. Every other meaning of the word, on a court, on a page, or in a column of numbers, is that same fixed first line doing that same job.
From the word’s history
Wiktionary analyses the word as base + line: the line that acts as the base for something else.
維基詞典把這個字分析成 base（底、基礎）加 line（線），也就是「給別的東西當底的那條線」。這正好對上它今天的用法：不論是地上的線還是報表裡的第一組數字，作用都是「當底」。
Wiktionary entry for 'baseline' (packet etymology_text: "From base + line.")
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