The difference between primary and secondary sources.
A primary source is information that you have been given first hand, or have taken first hand. For example, when you ask sample of people what their favourite fruit is out of apples and oranges, and they told YOU their answer, that would mean that no one can corrupt the answers and they cannot be misinterpreted.
A secondary source is when a person tells you the answers. For example, when you report on the results of a survey that was conducted by someone not connected to you. If in the example above, the interpretation of the data could be biased and make the survey sound like apples were the stand out winner, and you would have no idea that it was wrong because you haven't seen the raw data.