A segment performs the functions of a logical switch. A segment gives tenant network administrators the logical equivalent of a physical Layer 2 switch, allowing them to connect a set of VMs to a common broadcast domain. A segment is a logical entity independent of the physical hypervisor infrastructure and spans many hypervisors, connecting VMs regardless of their physical location.
Segments can be overlay or VLAN based depending on the traffic type parameter from the parent transport zone.
A segment is also known as a logical switch.
In a multi-tenant cloud, many segments might exist side-by-side on the same hypervisor hardware, with each Layer 2 segment isolated from the others. Segments can be connected using gateways, which can provide connectivity to the external physical network.
How to create a Segment in NSX-T

Networking > Connectivity > Segments > NSX
Add segment

We will want to provide a name for the segment
Connected gateway we will leave at none for now until we create a T1 in a later article
Select which transport zone
Assign a subnet
Save

Select no
Rinse and repeat for additional segments you need to create

Summary:
The base of creating segments is not that difficult. Figuring out how many segments are required, naming conventions, subnets, and security is the challenging part.