What does “double free” mean?

Question

As the title suggests I am new to C and have a mid-term coming up shortly. I am revising from past papers currently and a recurring theme is double free problem. I understan

Answer 1

Because free() will consolidate adjacent regions by managing the information stored in the tags before each region. It is something like managing the double linked list. So it would be dangerous if the buffer where ptr is pointing has been overwritten by an attack string, in which fake tags can be injected.