After putting the ground coffee in the basket, spread it out evenly. If the coffee is not leveled, the water will not go through the middle of the ground coffee but on the side of it. It will result in a not well-made extraction of coffee and will have an impact on the taste of the coffee.
The channeling is when the ground coffee is not evenly put into the porta filter and the density is not spread equally. The water is therefore not going to soak evenly the ground coffee and solid parts will remain. Due to channeling, espresso extraction would not be efficient.
If the tamping is not properly done or if the ground coffee is tamped too strongly, that would cause cracks and water flow passage due to a weak pressure. Accordingly, the parts with an excess of coffee ground will not provide the right amount of extracted coffee.
Sometimes the ground coffee could be compact and not evenly spread before tamping. If you tamp without leveling it and extract directly, the water will not properly go through the coffee.