Understanding Engineering jargons for Non-tech Product Owners
An entity in DBMS (Database management System) is a real-world thing or a real-world object which is distinguishable from other objects in the real world. For example, a car is an entity. An attribute of an entity gives us information about the characteristic features of an entity. Entity can be a real-world object, either animate or inanimate, that can be easily identifiable. 
For example, in a school database, students, teachers, classes, and courses offered can be considered as entities.
In relational databases, attributes are the describing characteristics or properties that define all items pertaining to a certain category applied to all cells of a column. The rows, instead, are called tuples, and represent data sets applied to a single entity to uniquely identify each item. 
An attribute is defined as a quality or characteristic of a person, place, or thing. Real life individuals and fictional characters possess various attributes. For example, someone might be labeled beautiful, charming, funny, or intelligent.
A primary key is the column or columns that contain values that uniquely identify each row in a table. A database table must have a primary key for Optim to insert, update, restore, or delete data from a database table.
For example, in the table below, CustomerNo, which displays the ID number assigned to different customers, is the primary key. CUSTOMERS. CustomerNo.
Definition of schema: Design of a database is called the schema. Schema is of three types: Physical schema, logical schema and view schema.
For example: In the following diagram, we have a schema that shows the relationship between three tables: Course, Student and Section. The diagram only shows the design of the database, it doesn’t show the data present in those tables. Schema is only a structural view(design) of a database as shown in the diagram below.
The design of a database at physical level is called physical schema, how the data stored in blocks of storage is described at this level.
Design of database at logical level is called logical schema, programmers and database administrators work at this level, at this level data can be described as certain types of data records gets stored in data structures, however the internal details such as implementation of data structure is hidden at this level (available at physical level).
Design of database at view level is called view schema. This generally describes end user interaction with database systems.
To learn more about these schemas, refer 3 level data abstraction architecture.
Definition of instance: The data stored in database at a particular moment of time is called instance of database. Database schema defines the variable declarations in tables that belong to a particular database; the value of these variables at a moment of time is called the instance of that database.
For example, lets say we have a single table student in the database, today the table has 100 records, so today the instance of the database has 100 records. Lets say we are going to add another 100 records in this table by tomorrow so the instance of database tomorrow will have 200 records in table. In short, at a particular moment the data stored in database is called the instance, that changes over time when we add or delete data from the database.
Variables are elements(data stored) that can take different values.