It helps to break it down:
a-sial-o-glyc-o-protein
"o" is just a combining phoneme; ignoring those, we're left with a (not) sial (sialic acid, a type of sugar acid that occurs at the end of saccharide (sugar) chains in biological systems) glyc (an oligo (several) saccharide chain) protein (I hope you know what this means).
Putting them back together we get "a protein bonded to a chain of several sugars that has had its terminal sialic acid residues removed".