if you don't want to take the interview now, you can ignore it. but i'd suggest you reply the email and say something like "i am not ready yet, but want to get back to you some time later". the recruiter will put you in some check list and will check back with you couple months later. if you want to talk to the recruiter now, just tell him/her your phone number and some time slot. he/she will ask about your background, your interests, and tell you the teams in google, the hiring process, and etc in the call.