A license key is a code generated by the Easysoft License server. It is a combination of the machine number and some other information; it is unique to a particular machine and cannot be used on other machines. A license cannot be activated until the license key has been applied.
If you are licensing online, you license key(s) will be applied without any intervention from you.
If you are licensing off line, your license key(s) will be emailed to you and you must apply them by double-clicking the email attachment (Windows) or appending them to /usr/local/easysoft/license/licenses (UNIX) ensuring that you remove any LIC: prefixes and there is only one license key per line in the licenses file.