Conversation view is a Gmail setting that allows you to set whether emails of the same subject are grouped together.

Having conversation view on allows you to see all emails for a specific email subject together. You do not have to open several email messages to follow the conversation or go searching for a message you may have already read and responded to. For example, replies to emails and forwards of emails make up conversations.
Here is an example of one conversation (which has multiple emails in it):

It is a personal preference whether to use this or not, but by default, Conversation view is turned on in Google Mail. You can turn this on or off in settings as described here:

You can learn more about how Gmail groups emails into conversations here.