What does a margin erosion mean?

I have income statements saying revenue and cost of goods sold increased at the same rates over some years, and heard this shows that there is a margin erosion.
Margin is the same as gross profit… but an increase in revenue and a corresponding increase in cost won't result in margin erosion if they're both increasing at the same percentage rate. Its only if they're both increasing in total dollars at the same rate that results in a lower margin.