In one sentence, describe what the following article is about:

Click and drag your cursor from the top-left cell of the data you want to group to the bottom-right cell of the data. It's in the left side of the green ribbon at the top of Excel. It's on the right side of the Data toolbar. This option is in the Group drop-down menu. Click Rows to minimize your data vertically, or click Columns to minimize horizontally. It's at the bottom of the pop-up window. Click the [-] button at the top or on the left side of the Excel spreadsheet to hide the grouped data. In most cases, doing this will only display the final line of the data. Click Ungroup to the right of the Group option, then click Clear Outline... in the drop-down menu. This will ungroup and unhide any data that was minimized or grouped previously.

Summary:
Select your data. Click Data if this tab isn't open. Click Group. Click Group…. Select a group option. Click OK. Minimize your data. Clear your outline if needed.