I don't know what this is [Part 3]

#Create a simulator object
set ns [new Simulator]
#Define a 'finish' procedure
proc finish {} {
    global ns nf
    $ns flush-trace
    #Close the NAM trace file
    close $nf
    #Execute NAM on the trace file
    exec nam out.nam &
    exit 0d
}
#Open the NAM trace file
set nf [open out.nam w]
$ns namtrace-all $nf

#Create four nodes
set n0 [$ns node]
set n1 [$ns node]
set n2 [$ns node]
set n3 [$ns node]
set n4 [$ns node]

#Create links between the nodes
$ns duplex-link $n0 $n1 2Mb 10ms DropTail
$ns duplex-link $n2 $n1 2Mb 10ms DropTail
$ns duplex-link $n3 $n1 1.7Mb 20ms DropTail
$ns duplex-link $n4 $n1 1.7Mb 20ms DropTail
#Give node position (for NAM)
$ns duplex-link-op $n0 $n1 orient right-down
$ns duplex-link-op $n2 $n1 orient left-down
$ns duplex-link-op $n3 $n1 orient right-up
$ns duplex-link-op $n4 $n1 orient left-up
#Call the finish procedure after 5 seconds of simulation time
$ns at 5.0 "finish"

#Run  the simulation
$ns run