This is a stunning admission of potentially criminal behavior by Trump. Why? He says he wanted Sessions to "end" the Russia probe. Well, Mueller investigated Trump's actions toward Sessions. He found "substantial evidence" that they crossed the legal threshold for obstruction.
The good people at @lawfareblog have a very handy collection of all the Mueller analysis on obstruction of justice. Here's what Mueller said about Trump's behavior toward Sessions, and how it met the threshold for all three elements needed for an obstruction charge. 
Finally, Trump got his facts wrong. He said Sessions recused himself from the Russia probe on his “first day” in office. That’s not right. Sessions announced the recusal on March 2, 2017, about three weeks after he was sworn in as Attorney General.
Here's an excerpt from the Mueller report. (Vol. II, p. 97) Mueller investigated Trump's actions toward Sessions and reached the conclusion that he was trying to get Sessions to "end" the probe into his campaign. They were right: Trump admitted it today in his tweet about #ALSEN.
