Heat Input per Unit Length
The thermal energy effectively delivered to the component per unit length of the weld bead during welding, which decisively controls microstructure formation and the extent of the heat-affected zone.
The heat input per unit length tells you how much energy you are putting into the part over a certain distance. This depends on current, voltage, and especially your travel speed. If you put in too much energy, the heat-affected zone gets too large and the grain structure can become coarse. With too little energy, you cool the weld too quickly, making the material hard and brittle, which can lead to cracking. You have to adjust your travel speed to the material and plate thickness. Better to do a test weld and see how the weld pool behaves than to blindly go by parameters.