The small glass square that goes on a wet mount slide is called a coverslip.

Here's why it's important:

* Protection: The coverslip protects the specimen from drying out and damage.

* Flattening: It helps flatten the specimen, making it easier to view under the microscope.

* Clarity: The coverslip creates a smooth, flat surface that allows light to pass through easily, improving the image quality.

How to use a coverslip:

1. Place a drop of water or mounting medium on the slide.

2. Carefully lower the coverslip onto the slide, at a slight angle to avoid trapping air bubbles.

3. Gently press down on the coverslip to ensure it's in contact with the specimen.

Important note: Coverslips come in various sizes, so make sure to use the correct size for your slide and specimen.