A summary judgment is a decision made by a court based on evidence and statements without going to a full trial. In the United States, the option is only available in noncriminal cases, and only in cases where there is no dispute over the material facts. (Impeachment is a political, not criminal, process, thus, it’s more akin to a civil trial than a criminal trial in the federal courts.) In civil litigation, one party, and sometimes both, can make a motion for summary judgment, and the district court judge presiding over the case can grant or deny the motion. If the judge grants the motion, he or she will proceed to render a judgment based on the law, without a presentation of evidence or a jury trial of the facts.
In the case of the upcoming impeachment trial, the “judge” who would initially consider such a motion would be Chief Justice John Roberts, but the “judge” who decides on the issues of law would be the members of the Senate, who would still fulfill their constitutional duty by voting to convict or acquit Trump. A conviction using this process would, as in a traditional impeachment trial, require a two-thirds vote.
Adopting a summary-judgment approach could work because, although it is true that the Constitution stipulates that the House of Representatives must pass an article of impeachment by a majority vote, and the Senate must “try” those articles of impeachment and vote to convict by a two-thirds margin to remove the president, the Constitution does not impose any particular trial rules or procedures on the Senate.