For the purposes of this debate, supporters of Longtermism believe that significantly more beings will be alive during the future, and that we should assign moral value to the unborn. Thus, they conclude that when evaluating our actions today, the largest consideration is how they affect the future.
Potential examples of Longtermist policies can include encouraging people to dedicate their lives to reducing long-run existential risks, allocating a substantial portion of the budget to try and improve the welfare of future generations, or trying to expand humanity to other planets.
Under a Longtermist view such policies are ethical even if they come at the very strong expense of more immediate and certain goals like poverty reduction, providing healthcare or increasing the happiness of the living.