Here's a breakdown:

* Pt: The chemical symbol for platinum.

* Arrow: Represents the direction of the chemical reaction.

* Catalyst: A substance that speeds up a chemical reaction without being consumed in the process.

So, seeing Pt over an arrow means that platinum is being used to accelerate the reaction, but it is not a reactant or product itself.

For example, the equation:

```

2H₂O₂ (aq) → 2H₂O (l) + O₂ (g) Pt

```

Indicates that the decomposition of hydrogen peroxide (H₂O₂) into water (H₂O) and oxygen gas (O₂) is catalyzed by platinum.