Financial Accounts are used to track financial information.
You should set up financial accounts in CiviCRM at Administer > CiviContribute > Financial Accounts before accepting donations. You will likely want to consult with your organization's bookkeeper to ensure that the accounts in CiviCRM match appropriately with those in the organization's books.
Normally organizations have some sort of electronic accounting or bookkeeping package such as QuickBooks, Simply Accounting, BYOB, AccPac, etc. The Chart of Accounts in that package is the basis for the Financial Accounts you configure in CiviCRM. Modify the financial accounts by adding new ones and renaming existing ones so that they match the accounts in your bookkeeping system.
If you want to export financial information from CiviCRM so that it can be imported into your bookkeeping package, then you need to pay particular attention to the Accounting Code field. The value in this field needs to match EXACTLY the code in your accounting package for the appropriate account. In addition, some packages such as QuickBooks use an Account Type Code that reflects its internal workings. This is slightly different than the Financial Account Type in CiviCRM, which just has the standard types of Asset, Liability, Revenue, Expense, and Cost of Sales. (Since CiviCRM is not designed to be a full-fledged accounting system, it has no use for Equity account types.) The Account Type Code will also need to match EXACTLY the account type in the accounting package.
Some accounting systems also require the Name of the financial account to match EXACTLY. In any case, this is a good practice.
A convenient way to ensure that you are setting up CiviCRM's Financial Accounts correctly is to use an export of the Chart of Accounts from your bookkeeping system. The account information shown on the screen can be a bit different - very frustratingly! - from what it exports and requires for import. If you are using QuickBooks, export to a .IIF file (on some versions of QuickBooks, use Files > Utilities > Export > Lists to IIF Files..., select Chart of Accounts and unselect all other lists, then click OK, select a location and click OK). If you are using a different package, choose to export the Chart of Accounts into a .csv (comma separated variable) file format if you are given several options, otherwise a txt format is good too.
Next open this file in a low-level editor like NotePad on a Windows machine or TextEdit on a Mac. The third line indicates the fields in the lines that follow:
!ACCNT NAME REFNUM TIMESTAMP ACCNTTYPE OBAMOUNT DESC ACCNUM SCD BANKNUM EXTRA HIDDEN DELCOUNT USEID
The lines that follow should have a format like this:
ACCNT Savings Bank Account (ING) 57 1182525763 BANK "2,994.62" 4004 N 0 N
From this you need to copy as follows:
NAME: "Savings Bank Account (ING)" into the CiviCRM Financial Account Name field
ACCNTTYPE: "BANK" into the CiviCRM Financial Account Type Code field
ACCNUM: "4004" into the CiviCRM Accounting Code field.
Let's try another example:
ACCNT Accounts Payable 66 1182527428 AP -448.75 4607 N 0 N
NAME: "Accounts Payable" into Name
ACCNTTYPE: "AP" into Account Type Code
ACCNUM: "4607" into Accounting Code
Easy, right? Well, this is the tricky part, so take you time. Reviewing these fields in QuickBooks Chart of Accounts front end will likely be helpful for you (e.g. Lists > Chart of Accounts in some versions of QuickBooks' menu).
You should do this for as many of the default Financial Accounts in CiviCRM as your bookkeeper indicates you need. Some organizations will not have to worry about Cost of Sales accounts because they don't use or maybe don't track the cost of Premiums, and others will not need to worry about the inventory for premiums. Many will want to create several different revenue accounts, and probably also separate asset accounts for their bank account(s), perhaps another for cash and checks received but not deposited, and one or more online payment processors.