The second line answers the first line's firmness with patience. Legge renders it as exercising forbearance with the ignorant, in which there will be good fortune, and the traditional reading attached to the line extends the image to a man capable of managing a household despite an untested son, tolerance for inexperience proving itself as a genuine capacity rather than mere permissiveness.
This is the central place of the lower trigram Kan, a position Chinese commentary treats as the seat of real capability even within a trigram defined by danger and difficulty. Where the first line corrects ignorance with firm boundaries, the second shows a different, complementary skill: bearing with continued inexperience without losing patience, trusting that instruction sustained over time will accomplish what a single sharp correction cannot.
Commentary tradition reads this line as a portrait of a household, or by extension any relationship of teaching, governed successfully despite an inexperienced junior member still finding his footing. The good fortune promised does not depend on the pupil already being capable; it depends on the teacher's own steadiness, forbearance exercised consistently rather than only when convenient. Read beside the first line, the two together sketch the full range instruction requires: firmness at the outset, patience throughout.