In the video why is the House waiting for the Senate to act first on immigration reform legislation

The House has a much stronger party leadership structure, since large assemblies of people require strong leadership to get things done.

By contrast, leadership in the Senate has been likened to "herding cats."

senators tend to be issue generalists, and as relative equals, senators tend to be less formal in their organizational behavior and more personal in their chamber communication and deliberation.

The House, on the other hand, is much more hierarchical, with power centralized in the party leadership.

What is the only branch of government that is able to initiate a reform bill?

Congress, as one of the three coequal branches of government, is ascribed significant powers by the Constitution. All legislative power in the government is vested in Congress, meaning that it is the only part of the government that can make new laws or change existing laws.

At which point is a bill sent to the Senate for approval?

SENATE ACTION After a measure passes in the House, it goes to the Senate for consideration. This includes consideration by a Senate committee or subcommittee, similar to the path of a bill in the House. A bill must pass both bodies in the same form before it can be presented to the President for signature into law.