If you are asking what HOA fees cover, it varies from building to building. In the larger buildings they usually have higher HOA fees because of higher expenses, maintaining amenities, and because a portion goes to building reserves for when components fail.
In smaller buildings the owners frequently opt to pay as they go in order to avoid high monthlies and leaving money on the table when they sell - any money put into reserve accounts stays there when you leave unless there is some written agreement to the contrary. There are arguments for both sides but that is a different question.
Typically HOA fees cover:
Homeowner's Insurance (for the building, you'll still need "walls-in" coverage for your belongings
Other items that can be included:
Use of facilities: gym, sauna, etc...
Heat (some buildings have boilers for the building)
If you contact the HOA they can give you a breakdown.
Lance King/Owner-Managing Broker