Daphne must choose to allocate her time between working (which provides money that she can spend on consumption) and leisure (which she enjoys for its own sake). She has $T$ hours per day to spend either working or participating in leisure. Consider the graph below, which depicts her indifference curves at three different levels of utility:
The straight red line is tangent to the $U1$ indifference curve at the point with $0$ consumption and $T$ hours of leisure. In other words, this particular budget constraint has a slope just low enough that Daphne decides not to work at all. If the slope were any steeper, Daphne would decide to work at least a little bit.