Symbol on silk screen of design
It is probably a screwdriver-adjusted "trim-pot" (potentiometer). Probably to adjust the sensitivity of the sensors to compensate for different operating conditions (ambient light, etc.).  It looks like the pot is on the OTHER side of the board from what we see in your photo.

You said: "This is the circuit diagram I used to make my line follower robot." but we don't see any circuit diagram. And then you said: "I don't have [a circuit diagram]".  So we are confused what you are doing with this board?  How can you design a PC board without the schematic diagram?