When you are given quotes to analyze, the first thing you should do is consider what the quote actually says. So, what does it say? It says, "King, we know what we are but do not know what we may be." Now, ask yourself, what does the quote mean? Ophelia means that we as humans (even perhaps women) know that we are human; however, we do not know what we will become. Let's face it no one knows what awaits us in the future. Finally, ask yourself, why does this quote matter? Why is it important? This part can be tricky because it depends on the context within the play. She says this following the death of her father. Again, you could view this several ways: Ophelia expressing her views on death; Ophelia expressing her views on the fate of women; Ophelia alluding to her own suicide.
I hope this was helpful.