In this section of the course, we cover different Data Types.
Apex uses the same primitive data types as SOAP API, except for higher-precision Decimal type in certain cases. All primitive data types are passed by value.
All Apex variables, whether they are class member variables or method variables, are initialized to null. Make sure that you initialize your variables to appropriate values before using them. For example, initialize a Boolean variable to false.
Resource: Apex Primitive Data Types - https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/langCon_apex_primitives.htm