By default, when you add most accounts to Outlook, your messages will be displayed organised into conversations. A conversation includes all messages in the same thread with the same subject line.
To turn off Conversation View in Outlook on Desktop:
From any mail folder, such as your inbox, select View > Show as Conversations and untick this option to turn this off.
You can then choose where to disable Conversation View in All mailboxes or just This folder for the current folder you are viewing.
To turn off Conversation View in Outlook on the web (OWA):
Click the Settings (gear) icon in the top right and then View all Outlook settings
Select Email on the settings menu on the left and select Show email as individual messages under Message organisation