An activity is represented in Moodle by a course. An achievement is represented by a badge.
How to award a badge (achievement)
Step 1. Click on "My courses" and select the one that corresponds to your activity.
Step 2. Under "Course administration", select "Manage badges".
Step 3. Click on the trophy icon under "Actions"
Step 4. Select "Module leader" from the list of roles
Step 5. Select the course (activity) participant to whom you wish to award the badge.