A % is a universal mathematical statement. The "old" SI units are, as stated above, mmol per mol. The first "m" means a "milli", which means a one thousandth. A "mol" is a special unit of measurement, pronounced "mole", so, unless scientifically trained, you can read it as, "a certain number of very tiny little bits of something in a bigger bit of something". The important thing is to get used to the numbers and what is high, what is low and what is acceptable. After all, none of us look in our wallets and think what percentage the contents are of the Gross Domestic Product, we just calculate if we have enough to see us through 'til Friday.
Sally