Topological sort
Linear ordering of a DAG's nodes such that for every edge u→v, u comes before v. "Dependencies first".
Two classic algorithms: Kahn's algorithm (peel off nodes with no incoming edges one at a time) and DFS-based (post-order, reversed). Both O(V+E). Doesn't exist if the graph has a cycle — then topological sort is impossible.
Powers build systems (Make, Bazel — which files should be built in which order), package managers (npm, apt — install order), task schedulers (Airflow, Dagster), course prerequisite checks, instruction scheduling in compilers.