[ns] ns code interpretation ?

[e_cherkaoui]

plz help !

what does it mean this code :

for { set i 1 } { $i < 5 } { incr i} {
    $ns at [expr $i + 0.5] "$app_(1) send 100 {$app_(0) recv {my message $i}}"
}

and the result : 
+ 3.5 6 7 tcp 140 ---A--- 0 3.0.0.0 5.0.1.1 201 9
- 3.5 6 7 tcp 140 ---A--- 0 3.0.0.0 5.0.1.1 201 9

thank u