One of the difficult things to get used to when trying to convert over to Outlook on the Web (from the Outlook client) is that all the various options, buttons, and settings are in different places. Microsoft is trying to make that easier by adding a new ribbon bar to Outlook on the web. This brings it closer to the user experience you’d find in the Outlook client. Here’s how that works…
The first time you use Outlook on the Web after this change makes it to your tenant, you’ll see this pop-up next to the drop-down arrow for the ribbon bar:

By clicking the drop-down arrow, you can switch between the Simplified ribbon and the Classic ribbon:

For mail, the Simplified ribbon just has the various menu words with drop-down arrows (where appropriate), and only contains a subset of the available menu options:

If you switch to Classic ribbon, you see more options along with the associated icons. Here’s the series of options for the Home menu:

And here’s the View menu options:

For your calendar, you get the same visuals. Here’s the Home menu options:

And here are the View menu options:

Do keep in mind… The features for Outlook on the Web and the Outlook client are not identical. Feature parity between the two continues to get closer, but there may still be things you do in the Outlook client that you can’t do in Outlook on the Web (and vice versa).