what does resume mean in file exists rules

I am very new to ftp and the software. I have a couple of questiions. First, in the file exists rules i understand most of it but what does it mean for resume? I know overwrite and skip are pretty understandable but not sure what it means to resume.
Second. in my queue i have a status that says waiting for children to finish. What does this mean?

>file exist rules
It is highly recommended to leave the pre-configured rules as is. Only in exceptional circumstances should they be changed.
>waiting for children
It means that some files belonging to the parent folder are not transferred yet.

I have an online storage site for my files and i use it as a backup and only want the changed files on my local drives to be transferred over. will the pre-configured rules work for this?
what does the software do when the rules are set to resume?

Yes, by default only the modified files will be transferred.
Resume will resume/restart broken transfers.