A company's liquidity is a measure of its ability to pay off its current debts, as you probably already know. There are different ways to measure it - not everyone agrees on the best method to do it. The 'Roll' and 'Zero trading days' are two methods. I searched these out and explaining them is a bit like rocket science. If you search for "liquidity roll" or "liquidity zero trading", you should be able to find what you are looking for.

They're referring to a paper published in 1984 by Dr. Rickard Roll. The [last name, publication year] is the short reference and then the full reference should be in the bibliography to the text.