‘Are you mad at me?’—’¿Estás enojada conmigo?’
‘I am’. In Spanish, the equivalent answer is ‘estoy’, right?
Not quite. The correct answer is ‘lo estoy’.
The lo in ‘lo estoy’ is called the resumptive lo. Like the last missing piece of a puzzle, without it, you can still see the whole picture, but it is still incomplete. In the same way, native speakers will most likely still understand the above answer without the resumptive lo, but it just won’t feel right to them.
The lo is resumptive in the sense that its use saves the speaker from having to repeat a clause in the immediate or recent sentence —’mad at you’. Note that the resumptive lo is grammatically required in Spanish.
Greg Hullender gave a detailed explanation of the resumptive lo in this blog post.