What is the significance of 'concrete type' when dealing with Julia errors?

Julia's type system is powerful, but sometimes errors occur because a type isn't 'concrete'. Concrete types have a fixed size and layout in memory (e.g., `Int64`, `Float64`). Parametric types (e.g., `Array{T,1}`) are not concrete until `T` is specified. Errors can happen if a function requires a concrete type but receives a parametric one. Use `concrete_type(type)` to attempt to make a type concrete.