Magbigay means to give (something not particular)
Ex.: Magbigay ka ng pera sa kaniya. - Give him/her money. (The money is not someone particular's money. We also don't know what type of money it is, whether it's coins or paper money.)
Ibigay means to give (something particular)
Ex.: Ibigay mo ang bag niya. Give him/her his/her bag. (The bag is his particularly.)
Also, the focus on "magbigay" is on the subject, so the object would not matter much.
But on "ibigay", the focus is on the object, so it has to be something known and particular.
Hope you understood and not get confused. :)