G'day there Becky, Golden syrup is made from sugar cane, and is like molasses, but much more refined. It is dark golden in colour and not as thick. It has more the consistency of honey. Honey would be a fairly good substitute, or if you have it over there, use light molasses as the dark one would be too heavy.
A slice is generally a biscuit (cookie) rolled out, then baked on an oven tray, in one piece, and cut after cooking.
Angel O:-)