[Solved] What is the meaning of the function na() in pinescript?

ctwhome Asks: What is the meaning of the function na() in pinescript?
I can’t find nor understand the funcion na() in pinescript. In the example below, the script prints a Heikin Ashi OHLC bars for a specific custom timeframe

strategy("[⚡TC⚡] Heikin Ashi Timeframes", overlay=false)

higherRes =input(title="Time 1", type=input.string, defval="90")

is_newbar(res) => 
    t = time(res) 
    not na(t) and (na(t[1]) or t > t[1])

o = security(syminfo.tickerid, higherRes, open)
h = security(syminfo.tickerid, higherRes, high) 
l = security(syminfo.tickerid, higherRes, low)
c = security(syminfo.tickerid, higherRes, close)

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