These are partially full urns (1% - 99% full) made by Woodcutting while having a strong woodcutting urn (r) in your inventory. You can only have one of these at a time, as a new urn won't start to fill while you have another partially filled urn of the same type in your inventory or bank, or when you have 10 full urns of that type.
This is collecting woodchips when I chop wood at or below Woodcutting level 75.
We are, as always, looking for people that want to help us out with keeping our guides and other RuneScape related information up to date. Jagex loves to make changes and we need help keeping up with those changes.
If you want to help us out register on the forums if you do not have an account already. Once you post enough to be promoted to an Established Member feel free to apply for Content Editor.