Have a look at Week 6 Lesson 1 and 2 to learn about measuring and comparing mass.
When we use standard measures for weight, we use grams and kilograms. Ask a grown-up if you can have a look in the kitchen for things that have their weight (how heavy they are) printed on them. You could look on cereal packets, flour, coffee jars, tins and margarine tubs.
In your book write a list of the things that weigh more than 500g and less than 500g.