Cardinality says how many rows on each side of a relationship can pair up. Its maximum gives the shapes 1:1, 1:N and N:M; its minimum is zero (optional) or one (mandatory). Crow's foot notation, introduced by Gordon Everest in 1976 and drawn by MySQL Workbench 524 , puts the maximum next to the entity (a bar for one, a fork for many) and the minimum behind it (a circle for zero, a bar for one). Read from one entity to the far end: "one order contains one or more items."
Orders and products are N:M, drawn here already resolved into the associative entity Order item and two 1:N lines. Categories form a recursive relationship. Cardinality comes from business rules, so ask the owner; no foreign key can enforce "at least one item," so PHP must.