The name of the floor level defined in your story data.
The total seismic weight of the structure is: $$W = g \times \sum_i=1^n m_i$$ where $m_i$ = U1 (or U2) mass of story i , and $g$ = gravitational acceleration.
ETABS generally "lumps" masses at the floor levels to simplify the analysis into a Multiple Degree of Freedom (MDOF) system. Mass - CSI Knowledge Base
The table typically displays the following values for each defined story: : The specific level of the building being reported.
To view the report after running an analysis, navigate to: > Show Tables > Analysis > Results > Structure Results > Mass Summary by Story . 2. Core Components and Columns