What is a plan during sprint planning meeting?

The Sprint Backlog can be thought of as the Development Team's plan for meeting the agreed Sprint Goal. It is essentially a forecast of work which they inspect and adapt throughout the Sprint so the Goal can be better met.

A Development Team should be able to explain how they are planning on doing all the work they have included in their forecast. Otherwise how would the Scrum Master and Product Owner have any ability to understand and look for impediments while the work is being done. Some people understand that better if you explain it as "The Product Backlog items selected for this Sprint plus the sub-tasks for delivering them is called the Sprint Backlog".

Oops, hit submit before I finished.   

The Guide has this in the definition of Sprint Planning

The Development Team usually starts by designing the system and the work needed to convert the Product Backlog into a working product Increment. Work may be of varying size, or estimated effort. ... Work planned for the first days of the Sprint by the Development Team is decomposed by the end of this meeting, often to units of one day or less. 

By the end of the Sprint Planning, the Development Team should be able to explain to the Product Owner and Scrum Master how it intends to work as a self-organizing team to accomplish the Sprint Goal and create the anticipated Increment.

That would be the plan referenced as part of the Sprint Backlog.