I'm so glad to receive a conditional offer Can't wait to embark on new career.

HR said that they will conduct background check and prepare me for the medical check.
So what is the background check about?
Only character check with my referee and HR of the previous company?

just wonder if I don't get a good word from ex-HR,will it cause me to fail the background check?

I had contacted my referee(ex-colleague).She had warned me about our ex-HR who is a big follower of my ex-boss，they get offened when employees resign...-.-"