They're referring to dynamically-sized arrays that do not store their own length. In C, this would be a pointer to the first element of an array; the length of the array must be passed around independently to use it safely. Instead, they're advocating collections which store their own length, such as vectors in C++ or arrays in C# or Java. Personally, I believe there is a need for three different kinds of array types:

1. Array types of fixed length, ensured by the type system. This corresponds to arrays in C, C++, and Rust, and fixed-size buffers in C#.

2. Array types of dynamic length, in which the length is immutable after creation. This corresponds to arrays in C# and Java, and boxed slices in Rust.

3. Array types of dynamic length which can be resized at runtime. This corresponds to vectors in C++ and Rust, and lists in C#, Java, and just about every interpreted language.

Of course, for FFI purposes, it is often acceptable to pass arrays as a simple pointer + length pair, this being the common denominator of sequential arrays across low-level languages.