Production of a resource is depicted by having the resource symbol in a brown production box. In this example the symbol means that you increase your plant production parameter on your player board 1 step. This in turn will give you a steady income of plant resources.

Production is not limited to 10; if your production exceeds 10, just leave the marker on 10 and start a new marker to complement it. Only M€ production can be negative (only as far as -5, though). But since TR is added to your production, your total income will not be negative.