Round to the neares thousand means round to the nearest 1,000.
As you have 12,386. To round it to the nearest 1000 you look the hundreds:
If the hundreds are greather than or equal to 500 you round to the next thousand.
If the hundreds are less than 500 you round to the same thousand.
As the hundreds of 12,386 are 386, less than 500. 12,386 rounded to the nearest thousand is 12,000