Among other things it shows his intent - until Don McGahn talked him out of it. There are many angles to this story. Also, Don McGahn said that "to the best of his knowledge" Trump did not ask anyone else to follow through on prosecuting Hillary and Comey. It is currently unknown whether Trump discussed this with Matt Whitaker before making him AG. There is much more to be known. The legal experts think this is a really big deal.
From the OP:
It is unclear whether Mr. Trump read Mr. McGahn’s memo or whether he pursued the prosecutions further. But the president has continued to privately discuss the matter, including the possible appointment of a second special counsel to investigate both Mrs. Clinton and Mr. Comey, according to two people who have spoken to Mr. Trump about the issue. He has also repeatedly expressed disappointment in the F.B.I. director, Christopher A. Wray, for failing to more aggressively investigate Mrs. Clinton, calling him weak, one of the people said.
Mr. Trump repeatedly pressed Justice Department officials about the status of Clinton-related investigations, including Mr. Whitaker when he was the chief of staff to Attorney General Jeff Sessions, according to a person with direct knowledge of the conversations. CNN
first reported those discussions.
During the presidential race, Mr. Whitaker, a former United States attorney, also said he would have indicted Mrs. Clinton, contradicting Mr. Comey’s highly unusual public announcement that he would recommend the Justice Department not charge her over her handling of classified information while secretary of state.
“When the facts and evidence show a criminal violation has been committed, the individuals involved should not dictate whether the case is prosecuted,” Mr. Whitaker
wrote in an op-ed in USA Today in July 2016.
The following CNN article also adds some background:
Washington (CNN) President Donald Trump on multiple occasions raised with Deputy Attorney General Rod Rosenstein and Matt Whitaker, who was then-chief of staff to Jeff Sessions, whether the Justice Department was progressing in investigating Hillary Clinton, according to a source familiar with the matter.
The President also wanted his previous White House counsel, Don McGahn, to ask the Justice Department to prosecute Clinton on numerous occasions, but McGahn rebuffed doing that, the source said.
Anticipating the question about Clinton would be raised, Whitaker came prepared to answer with what Justice was doing on Clinton-related matters, including the Clinton Foundation and Uranium One investigations, the source said. The source added that Whitaker was trying to appease the President, but did not seem to cross any line.
More: Trump raised investigating Clinton with Rosenstein, Whitaker and McGahn - CNNPolitics