A preferred return is a predetermined percentage that must be paid to the investor before the sponsor collects any of the distributable cash. After the preferred return is paid to investors, any remaining profit above that percentage is split between the investor and sponsor at a predetermined ratio.
A preferred return comes from the profits of the company and therefore is not a guaranteed return.  It is also not interest, which is an amount owed on a debt instrument.  How the preferred return is calculated depends on the agreement.  It can be a simple annual amount on capital invested or a compounding rate on all outstanding amounts.
Read the PPM carefully to understand the terms of the expected return and the waterfall (which investors get paid first). Loans generally take preference to preferred returns.
For example, a $100,000 investment into a syndication with an annual non compounding 6% preferred return would be a $6000 annual return until the project is finalized. If the project manager took 5 years to complete, the preferred return would pay out $30,000 plus the $100,000 investment. Once the total preferred return is paid, the remaining profit is split between the sponsor and investors, according to the offering documents.