Securities are units of value that can be aggregated and tracked in investment accounts.
Examples of securities include stocks, mutual funds and bonds.

All securities are recorded relative to your base currency but can be displayed relative to any other currency.

You can add a new security by navigating to Tools > Securities.

Alternatively, select the "Securities Detail" register of your investment account, and choose Add Security > New Security 
(Creating a security using this method will automatically associate it with the investment account you are currently working in)

There are a number of fields you will need to enter:

Security ID
A unique identifier of a security within Moneydance, it's not something that has any existence outside of Moneydance.

Security Name
The name of the security.

Ticker Symbol
Enter the symbol for the stock. Moneydance can access stock information from Yahoo! Finance USA, Yahoo! Finance UK, and Google Finance.

This value can be changed either from the "History" window or by using the extensions that automatically update prices.

Value Prefix and Suffix
By default, holdings are measured in shares as in equity holding. If you have some other kind of denomination for your holdings (such as contract for options) you can enter them here either as a prefix or a suffix to the price per unit of the security.

Be sure to enter the same number of decimal spaces that are used in your broker's share accounting (In most cases this will be 2, although some use 4 or 6.)

If you have already created your security, you can associate it with a particular investment account by opening the Securities Detail view and clicking on the “Add Security” button.

Select the appropriate security and fill out the rest of the security information identifying it as a stock, bond, CD, option contract, etc., and the particular information unique to that type of security.
Most of this information is just used for identification purposes and is not used in calculations of any kind within Moneydance.