¿Quién hizo lo siguiente?
?quién hizo lo siguiente? what does the "lo" mean? and why is it there? thank you
3 Answers
When siguiente is used as a noun, it is "el siguiente" or the following thing or the following one if we know what el refers to. When it is indefinite as to what it refers to, it is lo siguiente, but still means the following one or things (we just don't know yet what those things are).
The el or the lo are the things that follow. Think of siguiente as a adjective modifying the el or lo.
In English we use the present participle as a gerund or noun (following). Spanish doesn't do that so siguiente isn't (even though it isn't technically a gerundio) used as a noun by itself, they use the el (la) or lo to be the noun part (thing) and siguiente to be an adjective (following), but then call the whole pair el (la) or lo siguiente a noun (following thing).
At least that's a non-native's viewpoint. There may be some more abstract reason why the el/la/lo are there, but it seems to work for me when trying to understand some of Spanish's other uses such as the el que, la que, lo que relative pronouns. If rather that using the English which you look at it as the thing that it always seems to make sense to me.
The "lo" is taken as it. The direct translation is "Who did it, that which follows" which of course it not really the way english is spoken (Yoda speaks that way, but he's a Master Jedi and he can talk anyway he wants). So, in this sense, "lo" is "it". 
It's there because spanish is spoken in the way it's spoken and english is spoken in the way it's spoken and southern english is spoken the way it's spoken. It just is. Don't hang yourself on that...just know it and move on!
Who did it next?
Who went next?
Who was next?
I think those might be possible interpretations as well.
