This is very common, you memorize a path to a file or folder, you open up Vim and try to write what you've just memorized, but you are not 100% sure it's correct, so you close the editor and start over.
When you want the path completion feature,  and you have a file /home/ubuntu/my_folder/my_file and you are editing another file referencing to the path of the previous one:
Enter insert mode:
insert or do it the way you want.
Next, write /h. When the cursor is under h,
press Ctrlx and then Ctrlf so the editor will complete it to /home/ because the pattern /h is unique
Now, suppose you have two folders inside /home/ubuntu/ called my_folder_1 my_folder_2
and you want the path /home/ubuntu/my_folder_2
as usual:
Enter insert mode
write /h and press Ctrlx and then Ctrlf . Now you have /home/
Next add u after /home/ and press Ctrlx and then Ctrlf . Now, you have /home/ubuntu/ because that path is unique.
Now, write my after /home/ubuntu/ and press Ctrlx and then Ctrlf . The editor will complete your word until my_folder_ and you will see the directory tree so use the arrow keys to choose the one you want.