Mowing residues and dead leaves tend over time to create a thick impenetrable layer, called felt, which tends to absorb moisture, preventing it from reaching the grass roots, so it is best to remove this material, to allow fertilisers and water to reach the ground.
The operation of removal of the felt should take place every 4-6 months. To practice aeration use a special rake with long, narrow and parallel teeth, which anchor into the felt and lifts it. After having passed over the whole lawn with the aeration rake, remove the material using a normal rake or a broom for leaves. If the lawn to be aerated is very large you can also use a Scarifier machine, that will make the work easier.