Layers help filter physics and queries.

[SerializeField]
private LayerMask enemies;

Use in a Raycast:

Physics.Raycast(
    transform.position,
    transform.forward,
    10f,
    enemies
);

Get a layer by name:

int enemyLayer =
    LayerMask.NameToLayer("Enemy");

Use Project Settings → Physics to configure which layers collide with each other.