The TCP / UDP method is a packet flooder. It is NOT a SYN-Flooder!!
In the "message" you can set some payload to send to the targeted service.
If you need the newline chars, you can use \\r and \\n to construct the desired command / message.
If "Append random chars" is checked, 6 random characters are added at the end of the message.
"Wait for reply" waits until the packet is completely send. (you may want to check this on slower connections!)
"Failed" counts the unsuccessful connection attempts to the target.
In most cases 10 threads is more than enough and should use all available upload-bandwidth, however with the speed-slider you can adjust this.