When you have a very large number of child records associated to the same parent record in Salesforce, we call that “data skew”.

Types of Salesforce Data Skew
Account Data Skew
Certain Salesforce objects, such as accounts and opportunities, have special data relationships that maintain parent and child record access under private sharing models. If too many child records are associated with same parent object in one of these relationships, this imbalance causes something called “account data skew,” which in turn causes performance problems.

Ownership Skew
Similarly, when a large number of records with the same object type are owned by a single user, this imbalance causes something called “ownership skew.” Ownership skew also causes performance problems, which can surface when you’re managing your role hierarchy and sharing rules.

Lookup Skew
Lookup skew happens when a very large number of records are associated to a single record in the lookup object.  Because you can place lookup fields on any object in Salesforce, lookup skew can create problems for any object within your organization.