Still not enough information.
Link me where it says it and give more context.
Obviously it means they have give the question a code in advance, but need more detail to understand why and the significance.
Update
So have looked at it and this is Sociology and you are talking about research methods?
Precoding is where they have a question and there are a limited number of responses i.e choiceA, B, C etc They are useful because they can then be analysed as data in pre arranged groups.