This question is best answered by going to www.leatrix.com and checking the FAQ, but briefly what it does is change the frequency that your machine sends TCP acknowledgements to the other end of the connection. Consider a situation where the remote host is sending a large amount of data to your machine that will require the sending of multiple data packets. Rather than sending acknowledgement for each one, the default (and generally more efficient behavior) is to wait until multuple packets are received and ack them with a single packet. In certain specific cases, the remote host may require an acknowledgement to the first packet before it sends the next packet. The leatrix fix tells yout stack to ack each packet as soon as it's received.
A subsequent post asked if this was related to disabling Nagle (TCP_NoDelay). Leatrix deals with receiving data, Nagle changes the way data is sent from your computer, so the answer is no. By default, your TCP stack will hold on to an amount of data smaller than a full packet size before sending it to the destination is hopes that the application will send some more and the stack will be able to send a full packet, and thus make more efficient use of network resources. Disabling Nagle tells the stack to send any data sent by an application immediately regardless if it results a full packet or not.
So Leatrix affects how data is received, Nagle affects how data is sent.