To round a number to the nearest digit we must look at the digit to the right of the number we are rounding to.
In this case, since we are rounding to the nearest ones place, or units, we will look at the digit to the right which is the tenths place.
If the tenths place is 5 or greater the unit, or ones, will increase by one. If the tenths place is less than 5 our ones will stay the same.
Our tenths digit is a 3, therefore our units will stay a 7. The rest of the numbers to the right of the ones, disappear or become zeros but we don't place in the answer.
387.36 is between the 2 ones of 387 and 388.
Since the tenths was a 3, 387.36 will round to 387 to the nearest ones.