When you cast this spell using a spell slot of 3rd
level or higher, the damage increases by 1d8 for every two slot levels
above 2nd.
Spritual weapon is a 2nd level spell. The first part of this description is simply telling you the rules for casting it from a slot higher than it's level, i.e., third level or higher. Many spells have greater effects when they are cast with a higher level spell slot.
The second part tells you specifically what effect casting with a higher slot has and how the spell improves based on which spell slot you use. In other words, casting it from a 3rd level slot doesn't grant any benefit, because that is not two slot levels above second.
The spiritual weapon's damage works out as follows:
- 2nd level slot: 1d8 + your spellcasting ability modifier.
- 3rd level slot: 1d8 + your spellcasting ability modifier.
- 4th level slot: 2d8 + your spellcasting ability modifier.
- 5th level slot: 2d8 + your spellcasting ability modifier.
- 6th level slot: 3d8 + your spellcasting ability modifier.
- 7th level slot: 3d8 + your spellcasting ability modifier.
- 8th level slot: 4d8 + your spellcasting ability modifier.
- 9th level slot: 4d8 + your spellcasting ability modifier.