OFDM Insert Preamble and USRP N210

Hello I am Takuya Nakanishi.
I am a 4th year college student from Japan.

I have two questions.I want to send OFDM preamble signal.and I created
blocks are random source,OFDM mod,OFDM insert preamble and usrp sink in
a program. But, I don’t understand OFDM insert preamble block’s
parameters"Preamble and FFT Length".
Q1: What does two parameters indicate?
Q2: “In the attachment” OFDM insert preamble block’s flags.

I’m sorry.I weak English.