The Mass Action block allows instructors to perform actions upon multiple activities or resources, rather than having to perform repeated actions on individual items.
Step-by-step guide
From your course home page, enable Edit mode, and select Add a Block at the top of the blocks drawer on the right. From the Add a Block menu, click Mass Actions:
When you add the Mass Actions block to your blocks drawer and Edit mode is enabled, you will notice a small selection box appears to the left of each activity or resource in your course:
These selection boxes allow you to select multiple items and carry out any of the actions listed under the With selected... menu in the block. You can also use the top menu in the block to select all items within a section/topic, or select all items within the course, and then use the action available under the With selected... menu.
When your work is completed, from your course home page, disable Edit mode if you wish to keep the block (it will appear whenever you enable editing), or click the Gear icon at the top right of the block and select Delete Mass Actions block if you no longer require it.