Answer: It represents 1 thousandth
This is because the digit 1 is in the thousandths place value.
Here is the expanded form of that number
5678.321 = 5000+600+70+8 + 0.3+0.02+0.001
The 0.001 represents 1 thousandth, it basically means 1 out of 1000 or 1/1000.