A back up offer means the agent is looking for other offers. For some reason, he/she is not confident in the current buyer. I would have your agent contact the listing agent and find out what is up with this particular home and why they are looking for other offers. That may help you decide if you want to go through the trouble and write an offer or not.
I have seen many buyers get homes by means of a back up offer. So, find out first what is going on and then you will be able to make up your mind.
Joan Patterson, B.A., A.S.P., G.R.I., Realtor
Keller Williams Realty
8250 White Oak Avenue, Ste 102
Rancho Cucamonga, Ca 91730