The choice between either bread or a lily is an analogy Lionel Robbins (An Essay on The Nature and Significance of Economic Science, 1945) uses to describe the problem of scarcity that we face as individuals and society.
"The time at our disposal is limited. There are only twenty-four hours in the day. We have to choose between the different uses to which they may be put. The services which others put at our disposal are limited. The material means of achieving ends are limited. We have been turned out of Paradise. We have neither eternal life nor unlimited means of gratification. Everywhere we turn, if we choose one thing we must relinquish others which, in different circumstances, we would wish not to have relinquished. Scarcity of means to satisfy ends of varying importance is an almost ubiquitous condition of human behaviour"
Very broadly, Economics is the science that studies what best to do when we don't have enough of what we need or desire. Deciding where to allocate scarce resources can be very difficult but it is a reality to be confronted. Robbins describes the problem poignantly:
"There are cases when it is either bread or a lily. Choice of the one involves sacrifice of the other, and, although we may be satisfied with our choice, we cannot delude ourselves that it was not really a choice at all, that more bread will follow. It is not true that all things work together for material good to them that love God. So far from postulating a harmony of ends in this sense, Economics brings into full view that conflict of choice which is one of the permanent characteristics of human existence. Your economist is a true tragedian."