Your Inbox is where all your messages are downloaded to, unless you have otherwise specified (see Setting up Filters to learn about redirecting e-mail). When someone sends you an e-mail, it is displayed in a list fashion:
The first column represents the status of the message. The golden, closed envelope specifies that the message has not been read. Notice that all of the text in that row (of the first message) is also bolded. The in the second message row is white and opened. This means that you have read that message. Also take note that the text in the second message row is in plain format (not bolded). You can change the status of the message by clicking the envelope next to the message you wish to change. If the status is Read, it will be changed to Unread, and vice-versa.
The second column is the Date of which you received the e-mail. The date format, in this case, is MM/DD/YY (see Configuring Display Settings to learn how to change the date format). It also tells you the time at which you received the message. In this case, it is in a 24-hour format. This can be changed in the Display Settings Configuration.
The Sender is who the e-mail is from. You can click the name to compose a message to that person (see Message Composition).
The Subject is the topic of which the sender had specified. This gives you a brief, explanatory meaning of the message. By clicking the Subject of the message, you will be redirected to that message. For example, if a user clicked How's it going? (see above diagram), he/she would be redirected to that message, and be able to read the message, in full.
The Size of the message is the virtual size that the message takes up on the server. This is reported in Bytes.
The Checkbox in the last column is for message selection and actions. By clicking one of the checkboxes, a small Checkmark will appear in the box. Clicking it again removes the checkmark. By clicking the checkbox utmost to the top (directly right of "Size"), all of the checkboxes below it will be selected. This tool is for performing actions on selected messages. More will be discussed on this topic in Using the Toolbar.