The segment is a filter or categorization of a contact list, you can create different sub-list or filter it based on multiple filter criteria.
For example, you have a contact list of 1,000,000 subscribers and you want to create sublist out of this for Gmail IDs, so with the help of segment, you can create a Gmail segment which will filter all the Gmail IDs inside your contact list.
You can give any filter condition as follows.
And now you can send emails to this newly create Gmail segment only instead of the whole list.
You can create any filter based on any conditions such you want to filter all the contacts belong to the USA so you can create a segment based on country column i.e Country = USA and only send email to USA users with help of this segment option.
Kindly Note: You must have those columns in your list in order to create a segment for that such as a Country column.