My school attendances bad,What will happen if i have another day of?

My school attendance is really bad,My mum got a letter home from the school saying that when i have a day of school i have to provide evidence (Doctors note,Letter E.g) If i don't provide evidence they said that my absence will go down as unauthorised and would result in a referral being made to the Education welfare service(<-What does that mean) Also i took today of because i wasn't feeling well and i want to have tomorrow of as well but my mums saying that she will get a fine,Is this true? What will happen if i have another day of without a doctors note?