Learn in the textbookStructured Diagrams for Cause Analysis, Control, and Systematization
DefinitionA tree diagram (系統図) breaks an objective you want to reach into the means that achieve it, then splits each means into finer means, branching level by level.
ExplanationA means at one level is the objective for the level below, so asking "how do we do that?" again and again keeps the branches growing. You stop when the branch tips become concrete actions that can be assigned an owner and a deadline and started. The result lists every measure that could serve the objective and gives you material for deciding which to carry out first.
AnalogyIt works like a goal breakdown sheet: "speak English" splits into "get more chances to talk" and "build vocabulary", then goes down to "one online conversation a week".
Memory hookIn English it is a tree diagram: the trunk is the objective, and the branch tips are means you can act on tomorrow.