Relation
also: relations, relationship, relationships, one-to-many, many-to-many
A connection between two kinds of data, like "each customer can have many orders" or "each order belongs to one customer".
A link between tables, usually implemented with foreign keys: one-to-one, one-to-many (a user has many posts), or many-to-many (through a linking table). ORMs call these "relations". In formal database theory, "relation" also means a table itself.
A family tree. Each line says how two people connect: parent and child, siblings, spouses. The people are the data. The lines are the relations.
How data connects decides which features are easy. "Can a booking have more than one guest?" is a relation question. Getting it wrong early forces painful changes to live data later.
Right now Recipe has a one-to-many relation with Tag, so each tag can only belong to one recipe. I'd change it to many-to-many.
Before adding team accounts, explain the relations between users, teams and projects, including whether a user can belong to more than one team.
Assuming the app just knows two pieces of data belong together. The connection has to be designed and stored, usually as a foreign key.