As a condominium owner you have bought a portion of a corporation. When there are expenses that come up that the reserve fund is not high enough to handle, the cost is transferred to all of the owners (this is regardless of whether or not your unit has an issue). Examples might be if the exterior needs to be updated, new fencing, balcony issues, roofing, etc. There may be quite a long process involving estimates, discussions, etc. The total cost is divided up between all the units. Typically it will be apportioned based on the size of the units. An invoice will be issued for payments along with a payment schedule. This is not an optional payment. The amounts can be as small as $1000 per unit or can end up being very large sums of money. It is just like if you own a house. If there is a repair that needs to be done, you will have to pay to get it fixed.
The most important thing is to know what you are buying. You can't always predict what it going to come up but knowing what is currently happening in the complex is important.