From http://eel.is/c++draft/range.range#itemdecl:2:

template<class T>
 concept safe_­range =
   range<T> &&
     (is_lvalue_reference_v<T> || enable_safe_range<remove_cvref_t<T>>);

Given an expression E such that decltype((E)) is T, T models safe_­range only if the validity of iterators obtained from the object denoted by E is not tied to the lifetime of that object.

[ Note: Since the validity of iterators is not tied to the lifetime of an object whose type models safe_­range, a function can accept arguments of such a type by value and return iterators obtained from it without danger of dangling. — end note ]

Given std::vector<int> x{1, 2, 3};, if E is x, its type std::vector<int>& models safe_range, but the validity of the iterators obtained from the object it denotes are tied to its lifetime.