A class is considered coupled to another one if it uses it. Classes that are coupled to many other classes are said to have a high coupling. High coupling classes are tailored for a single specific use case, and will not be reused elsewhere. It is preferable to split those large monolithic classes into a few smaller modular ones.
The following code illustrates this rule:
class Foo // Non-Compliant - depends on too many other classes { T1 a1; // Foo is coupled to T1 T2 a2; // Foo is coupled to T2 T3 a3; // Foo is coupled to T3 T4 a4; // etc. T5 a6; T6 a6; T7 a7; T8 a8; T9 a9; T10 a10; T11 a11; T12 a12; T13 a13; T14 a14; T15 a15; T16 a16; T17 a17; T18 a18; T19 a19; T20 a20; T21 a21; }