What is the meaning of "ONT"?

Hi!

Welcome to the AT&T community!

ONT means Optical Network Terminal. It is a large white/gray box that looks pretty much like your regular NID (network interface device) usually mounted on the side of a house. For non-fiber connections, telcos use the NID to provide landline and internet service to the premise. 

If you are to receive an ONT, this means that your location qualifies for the network type called Fiber-to-the-premise (FTTP) and instead of a NID, you will be using an ONT. This means that AT&T will run fiber lines up to your home (on the ONT). The ONT converts "light" data transmission (inside the fiber cable) to electrical signal that can pass through copper wires inside your home (like ethernet cables). 

If you have an ONT/FTTP service, then the technician will need to run ethernet (RJ45) cables as inside wiring to reach the AT&T modem.

In theory and in application, data travels more efficiently as "light" inside the fiber lines compared to electrical signals in copper wires; hence, FTTP connections can support up to 75Mbps+ speeds or even 100Mbps if I remember it right.

Optical Network Terminal.  It converts the incoming fiber optic signals so that it cam be carried over an ethernet (or is it twisted pair) connection (for uverse it would be to the gateway/router).   For this , as well as other similar questions, I suggest (a) using google search (this for example) and/or (b) look at the FAQ section in dslreports.