Not all emails are worth reading. Some you do not want and some you just can not. You can find this trick to quickly mark messages as readable without opening them in Gmail.
How to mark emails as read in Gmail on the web
If you use the Gmail website on your Windows or Mac computer, you can mark all or individual emails as read.
To mark a single email as read, hover over the email from your inbox, then select the "Folder" button.
The email will now appear faded and the number of your unread emails will decrease.
To mark multiple messages as read, use the "Select" tool at the bottom left of the email list. Click on the "Checkmark" icon for all the messages you want to mark as read.
If you want to highlight all the emails displayed on the screen as read, click on the "Checkmark" icon from the top toolbar. All messages on the current page will be selected. Use the "Select All Conversations" option to select all conversations in the Inbox or category.
After making the selection, click the "Mark As Read" button from the top toolbar. This will mark all selected emails as read.
To mark messages as unread, you can select the emails and select the "Mark As Unread" button from the toolbar.
How to mark emails as read in Gmail for iPhone and Android
Gmail makes it incredibly fast and easy to mark your messages read directly from your Inbox. It's the same method for the iPhone, the iPad or your Android device.
Open the Gmail app and go to your Inbox. Then tap the sender's photo for an email you want to mark as read. You can select multiple emails in the same way.
After selecting email, click the "Folder" icon on the top toolbar.
Emails will be marked as read.
If you want to mark the messages as unread, select the messages again and click the same "Folder" icon.
Source of information: howtogeek.com