Why use websocket over http?
A webSocket is a continuous connection between client and server. That continuous connection allows the following:
Data can be sent from server to client at any time, without the client even requesting it. This is often called server-push and is very valuable for applications where the client needs to know fairly quickly when something happens on the server (like a new chat messages has been received or a new price has been udpated). A client cannot be pushed data over http. The client would have to regularly poll by making an http request every few seconds in order to get timely new data. Client polling is not efficient.
Data can be sent either way very efficiently. Because the connection is already established and a webSocket data frame is very efficiently organized (mostly 6 extra bytes, 2 bytes for header and 4 bytes for Mask), one can send data a lot more efficiently than via a HTTP request that necessarily contains headers, cookies etc…
what is full duplex communication?
Full duplex means that data can be sent either way on the connection at any time.
what do you mean by lower latency interaction
Low latency means that there is very little delay between the time you request something and the time you get a response. As it applies to webSockets, it just means that data can be sent quicker (particularly over slow links) because the connection has already been established so no extra packet roundtrips are required to establish the TCP connection.
For a comparison in what’s involved to send some data via an http request vs. an already established webSocket connection see the steps listed in this answer: websocket vs rest API for real time data?
These other references may also be useful:
Server-push whenever a function is called: Ajax or WebSockets
For a push notification, is a websocket mandatory?