When we talk about AOT then first terms which comes in minds , its View Engine. View Engine introduced in angular 4, which decreased the size of generated code using AOT. AOT is angular compiler and its responsible to convert HTML and TS code into efficient java script code during build phase.

Typescript files converted to Javascript files on compilation stage itself

Ahead Of Time compilation. Angular compiles components and templates to native JavaScript, HTML. It will cut down rendering time.