The by keyword sets up a "delegate": an object that will serve as an intermediary for your property.

by lazy allows you to declare the contents of a val via a lambda expression. When the property is first read, the lambda expression is evaluated and its result is used as the initial property value.

You can learn more about this in:
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Abstract val Custom Getter Abstract val Properties Abstract var lateinit Properties lazy Properties Overriding Properties Property Initialization in Declaration Property Initialization in Init Block
Delegates.observable() Delegates.vetoable() lazy Properties Map as a Property Delegate
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