means of making a living; source of income; bread and butter
Idiomatic expression literally meaning 'seed of rice/meal', used to refer to one's livelihood or the source of one's income. Often carries a slightly colloquial or down-to-earth tone.
This job is my bread and butter.
He makes a living from translation.
From 飯 (めし, 'meal, food') + の (possessive particle) + 種 (たね, 'seed'). Literally 'seed of a meal', metaphorically the source from which one's food (livelihood) grows.