Baseline is actually a reference line 

baseline is a line between two points on the surface of the earth and the direction and distance between them. In a triangular or square network, at least one baseline must be measured between two stations to calculate the size of the triangle or square.

Here check this diagram below 👇 

The line A B is a base line this line is an imaginary line we only use the points A and B 
First of all we put the point A and point B data in the total station machine to use as a reference line
Then we can mark any point like point E,F and C shown in the diagram above ☝️ 
And the line A C B is the survey line