Restricted stock or restricted securities are securities that are not freely tradable on the public marketplace.
You typically acquire restricted securities in a private sale from an Issuer before the company is public. Alternatively, restricted securities may be acquired through a corporate reorganization or acquisition, in return for services, or purchased from an original shareholder.
Restricted stock will usually have a legend printed on the stock certificate or statement stating that the shares cannot be sold or transferred without either registration under the Securities Act of 1933 or an exemption from such registration.
For more information about restricted securities from the SEC, please refer to the SEC websites here.