With filters you can create rules for incoming emails. You can for example add labels or star the incoming emails depending on where they came from, or you can archive, delete or forward the emails without any manual work.
Creating a new filter starts from the Gmail search box:
- Click on the downward facing arrow on the right side of the search box
- Add conditions for the filter from the advanced search box
- Click on the Create filter from the bottom of the box
- Choose what you want to do for the emails matching the conditions
- Click Create filter again
You can modify the filters from Gmail's settings under the Filters and blocked addresses tab.