to begin an impeachment inquiry - the constitution provides the House with the power to impeach.
The (republican) rule change gives the majority sole power to subpoena.
"During the Clinton and Nixon impeachment inquiries, the House passed their inquiry resolutions so they could gain tools like more subpoena power and depositions, and included in those resolutions were nods to bipartisanship that gave the minority party subpoena power, too.
But the House rules have changed since the last impeachment of a president more than two decades ago. In this Congress, the House majority already has unilateral subpoena power, a rule change that was made when Republicans last controlled the House, so Democrats don't need to pass any resolution to grant those powers."
https://www.cnn.com/2019/10/08/politics/nancy-pelosi-letter-impeachment/index.html