This generally refers to any concessional contribution which is claimed as a tax deduction by the contributor, and includes all employer contributions, and personal contributions claimed as a tax deduction by eligible individuals.
Deductible contributions will generally have up to 15% tax deducted by the fund. The super fund is then required to remit this tax to the Australian Tax Office (ATO) in its annual income tax return. For example, if the contribution was $1,000, the tax applicable to the contribution would be $150 and the net after tax contribution in the fund is $850.
An individual who has income (including some super contributions) over $250,000 may be required to pay an additional 15% tax on these contributions (known as Division 293 tax). This is paid personally following the lodgement of their tax return. If you are on very high income you should seek tax advice to see if this rule will affect you.