Hi Jonathan,
"Preocuparse" is a reflexive verb. "Preocupar" mean to worry or bother. "Se" refers to yourself or oneself. When you PREOCUPAR...SE, you are worrying yourself or preoccupy yourself about something. If you use the tú form, then you worry yourself as TE. If I am worrying, then I am preoccupying myself "ME" with something.
Thus:
ME preocupo - I preoccupy MYSELF
TE preocupas - you preoccupy YOURSELF
SE preocupa - he preoccupies HIMSELF
NOS preocupamos - we preoccupy OURSELVES
OS preocupáis - You all preoccupy YOURSELVES
SE preocupan - They preoccupy THEMSELVES
I hope this answers your question.