Links between tasks represent the sequence in which they will occur. For example; the successor task 'Boil Water' will first depend upon the completion of the predecessor task 'Fill Kettle'. Links can have a number of dependency types. Create links by selecting two tasks (with click and CTRL+Click), followed by the Link Tasks button.
SOURCE; microsoft project glossary-projectlearning.net
A dependency between tasks that specifies when a task begins or ends relative to another task. 'See Logical Relationship.