What does it mean when an employer says “you will get a raise if you do good”? Would it be weird if i renegotiate?

I went to a job interview today and i'm really bad at negotiating pay rate. When the employer told me id be getting paid 11.50 (which is only 50 cent higher than my current retail job) i gave her a look, and then she quickly stated that it would increase based on my performance. I accepted. I know i'm overthinking it, but does that mean if i start out good ill get an increase or will i have to work a year to see an increase.

P.S. I was contacted by the employer for a different position than I applied for. I accepted it, thinking it was going to pay the same (The starting rate for the position I previously applied for was 12.50 to 16) SHOULD I RENEGOTIATE OR JUST DEAL WITH IT ? HOW WOULD I GO ABOUT RENEGOTIATING

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