This is the 312th most frequent Spanish word.
muestra
Sample.
In this context, 'muestra' refers to a specimen or sample taken for examination.
La muestra del tejido será analizada en el laboratorio.
The tissue sample will be analyzed in the laboratory.
Here, 'muestra' is used to signify a demonstration or indication of something.
Esa acción es una muestra de su valentía.
That action is a proof of his courage.
In this case, 'muestra' means a small quantity of a product used as a demonstration or for testing.