noun
beginning of a line
Refers to the start of a line of text, code, or writing. Common in computing and document editing contexts.
行の先頭にカーソルを移動してください。
Please move the cursor to the beginning of the line.
このエディタでは、行の先頭にインデントを挿入できます。
In this editor, you can insert an indent at the beginning of the line.
先頭 is a more general term for 'head' or 'front' of something, while 行の先頭 specifically refers to the start of a line.
Compound of 行 (line) + の (possessive particle) + 先頭 (head, beginning). The exact historical derivation is straightforward; the phrase is a transparent combination of its parts.