Simply put, you and the bank jointly own the property in specific percentages, then you gradually buy the bank's share until you own it completely.
You typically pay for the use of the bank's share during this period.
To properly evaluate the offer, understand the cost of purchasing the shares, the rent, the expenses, and what happens if you decide to sell or pay off early.
The figures, fees and limits in this answer vary by bank and change over time — confirm them with your bank or the official source before acting.