FTP stands for File Transfer Protocol.
When we ask for FTP details we are asking for a way to log into your server so that we can access the application files for a support request, or upload the files for an install request.
When you sign up for a hosting plan you should have received an email containing information such as how to login to your control panel and details on how to login to your server.
The FTP details will comprise of a server address (eg. ftp.yourdomain.com), a username and a password. If you don't have them or don't know what they are, you can simply contact support team/hosting provider and they will give you this information.