Values used in the expression can be from component attributes, literal values, booleans, and so on. For example:
{!v.desc}
In this expression, v represents the view, which is the set of component attributes, and desc is an attribute of the component. The expression is simply outputting the desc attribute value for the component that contains this markup.
If you're including literal values in expressions, enclose text values within single quotes, such as {!'Some text'}.
Include numbers without quotes, for example, {!123}.
For booleans, use {!true} for true and {!false} for false.