Select the row(s) and/or column(s) you want to keep visible
Go to the View tab
Click Freeze Panes
To freeze only the top row, select Freeze Top Row
To freeze only the first column, select Freeze First Column
To freeze a specific row and column, select the cell below and to the right of where you want the freeze to start, then choose Freeze Panes
To unfreeze, go to the View tab, click Freeze Panes, then select Unfreeze Panes