To keep the answer simple. A one state solution for the Israeli and Palestinian question is to unify the separated Jewish State with the Palestinians as one country shared by all the respective religious groups.
Meanwhile a Two state solution refers to the idea of keeping both Israel and Palestine separate while allowing recognition by the UN of both states. As the state of Israel and the State of Palestine.
I know it is impossible but I would prefer a one state solution with a Parliament that is shared by all of the areas populations including Muslims, Israelis, and Christians.