I consider this quote from the wikipedia article to be helpful:
"In various versions of the C and C++ programming languages, an inline function is a function upon which the compiler has been requested to perform inline expansion.
In other words, the programmer has requested that the compiler insert
the complete body of the function in every place that the function is
called, rather than generating code to call the function in the one
place it is defined. Compilers are not obligated to respect this
and also this:
"Inline expansion is used to eliminate the time overhead when a function
is called. It is typically used for functions that execute frequently."