Hi!
normal secretions means when you don't need to cough to evacuate them. Lungs mucous is produced constantly to glue and carry away the small inhaled particles. They are mobilized by the air you exhale and mainly by the mobile hair of the epithelium (free translation from the french "cellules ciliées bronchiques", sorry) that act like an escalator. Then you swallow and don't notice. Smoking occurs a palsy of these hair and you need to cough even if you don't have too much to evacuate.

The increase of secretion might come from 2 factors:
- infection, need to evacuate more dead cells
- inflammation, increase of secretion in reaction to mechanical or allergic irritation.

You'll find a lot of books or abstracts on this subject for more details.