I want to understand what does domain function do in d3.js?

So you're probably working with a file that contains data. In the video the example data includes the highest temperature recorded every day for a period of time. For example the highest temperature recorded on 01/08/1973 was 294 degrees, and on 02/08/1973 it was 300 degrees.

The purpose of the domain function is to map the lowest and highest of those values to the visual range on a graph (which you will have already specified using the range function). For example, if the highest record temperature of the day in that time period was 100 and the lowest record temperature of the day was 0, the domain function would take those two values as an array. e.g:

//This assumes you are mapping temperatures to the y axis, and probably the date to the x axis.
yScale.domain([0, 100]);

Lets say the y axis range (in pixels) was from 0 to 100 e.g:

//Usually the highest number would be placed first for reasons explained in the video but for simplicity's sake I've done this.
yScale.range([0, 100]);

The domain function would then map the data values to the range. The element with the value of 100 degrees would be placed at position 100 on the graph. The element with the value of 0 degrees will be placed at position 0. An element that had a value of 60 degrees would be placed at position 60 etc.

I hope this helps and I haven't over complicated it...