SELTEXT is one of the fields that are linked to the texts of the grid columns. This particular one is responsible for displaying the name of the column in the change/display of layout popup.
Figure 79 ZDEMO_AIN_CL33 Column description in the layout popup
In case the TOOLTIP field is empty, the value from SELTEXT will be also used as tooltip and will appear when you hover the mouse over the column in the grid.
Figure 80 ZDEMO_AIN_CL33 SELTEXT as a TOOLTIP