How is a 2 pass-assembler different from a one pass assembler in resolving the future symbols?

These are 2 questions that I don't understand:

1. How does the One-Pass Assembler resolve the future symbol problem?

2. How is Two-Pass Assembler different from the one pass assembler in this respect?

   Does it resolve it in the first pass or the second pass? If it does it in the second pass,where does it actually differ from the one-pass-assembler? If it does it in the second-pass why doesn't it do in the first pass?