The most straightforward understanding of the phrase above is presented by Abraham Ibn Ezra, who says that it means that God instructed Moses and Aaron to go to the Israelites and prepare them for the Exodus that was due to take place. Rashi advances a different interpretation and says that it means that God forewarned Moses and Aaron that the Israelites were going to be a difficult nation to lead (most probably because of the slavery that they had endured) and that they should treat them with a certain amount of patience and indulgence.
A far less literal, but much more thought-provoking, interpretation appears in the Jerusalem Talmud, which states that God told Moses and Aaron to instruct the Israelites about the mitzvah of freeing slaves (Rosh Hashanah 3:5). This is strange, because as a nation of slaves they wouldn't have owned any slaves that ought to have been set free.
It seems likely that, according to this interpretation, the realities of mitzvah of freeing slaves were less relevant to the Israelites than the ethics of the mitzvah. As they stood on the cusp of freedom, they were being reminded that freedom isn't just something for them to dream of for themselves, it is also something to aspire to for others. And while this is true in a general sense for all of society, it is particularly true for those who have experienced the traumas and indignities of slavery themselves.
Before they went free, God wanted to remind them that they shouldn't allow others to suffer in the way that they had suffered. It was a timely reminder to them that they should ensure that their experiences of both slavery and freedom should inform the way that they treat other people in the future.