Simply put, recycling is contaminated when the wrong items are placed in the recycling bin.
This includes items that belong in a different recycling stream such as e-waste, garden organics and materials that are not currently recyclable like used pizza boxes and textiles.
Other examples include placing a glass bottle into a mixed paper recycling bin, or when materials are not properly cleaned and emptied, when food residue remains on a plastic yoghurt container or the cap is not removed from your rinsed plastic soft drink bottle.
If a bin is contaminated then the whole bin cannot be recycled and effectively turns your recycling into nothing more than landfill. So if in doubt check before you recycle!