Congratulations on the invite! This means the client likes your profile and has invited you to apply for their job.
All you need to do now is just follow the link and apply for the job. Please list all your relevant experience and answer any questions asked by the client in the brief or the job invite.
Should you be successful, the client will book you on the project. An invite is not a booking, but just the first step in the process to finding the right person for their project.