EDIT: Schroeder pointed out that I've mis-read the quote (due to lack of context). This is a corrected answer
The full quote is this
The graphs on this page gives an indication of how good the raw random data used by RANDOM.ORG is. Any source of raw randomness may be biased (skewed) towards 0 or 1, meaning that more 0s occur in the data than 1s or vice versa. An important part of generating true random numbers is to perform skew correction on the raw random data, i.e., remove any such skew. The graphs on this page show how close to unskewed the raw random data used by RANDOM.ORG is. If this data is perfectly unskewed, even before process, you will see a purity rating close to 100%. If there is skew in the data, you will see a lower rating. However, even if the raw data is skewed, the skew correction algorithm will correct it, and the skew will not appear in the processed data.
The first sentence
Any source of true randomness may contain skew towards 0 or 1 in the data
means that isn't a 'fair' RNG, which has equal probabilities for both outcomes. Instead, it's sometimes more likely to produce one number than another.
This does not affect the final generated data, because
However, even if the raw data is skewed, the skew correction algorithm will correct it, and the skew will not appear in the processed data.
So the data RANDOM.org presents to you is unskewed.