What does Python's eval() do?

The book that I am reading on Python repeatedly shows code like eval(input('blah')).

How exactly does this modify the result from calling input?

_{See also: Why is using 'eval' a bad practice? to understand the critical security risks created by using eval or exec on untrusted input (i.e.: anything that is even partially under the user's control, rather than the program's control).}

_{See also: How can I sandbox Python in pure Python?. The short version is that doing this properly will always be harder than choosing a proper tool instead of eval or exec.}

_{See Using python's eval() vs. ast.literal_eval() for a potentially safer technique.}

_{See How do I use raw_input in Python 3? for background context on why a book might have contained code like this, or why OP might originally have expected an input result not to require further processing.}