Copy-on-Write, is a resource management technique used in computer systems(mostly operating systems) to efficiently copy resources.
It is applied by making certain pages of the memory read-only, and only keeping a reference to the page. When a parent process creates a child process, both processes will share the same pages in memory, and those pages will be marked as copy on write.
By marking the pages copy on write means that if a process wants to make modifications to the page, then a copy of the page is made, and the process writes on the copy, thus not interfering with the other process.