In Google Sheets modification of rows, cells, and columns can help personalize your file. Learn how here.
Whenever you have too much cell content to be displayed in a single cell, you may decide to wrap the text or merge the cell rather than resize a column. Wrapping the text will automatically modify a cell's row height, allowing the cell contents to be displayed on multiple lines. Merging allows you to combine a cell with adjacent empty cells to create one large cell.
To unmerge a cell, click the drop-down arrow next to the Merge cells button, then select Unmerge from the drop-down menu.