If this is part of a problem you must solve, like a practice after learning something, it is Ok.
Repitiendo este proceso muchas veces, y determinando la proporción de éxitos para cada muestra, ilustra la idea de la variabilidad de muestra a muestra en la proporción muestral.
The first part is telling you what you are going to do, you'll repeat the process you learn (that I asumme is a part of the text you didn't write) and after doing it, it indicates you need to write on your own words "Ilustrar"- what variability from sample to sample,in the sample proportion, means.
Why use Ilustra? Why not Ilustrar?. It is because is an instruction so the verb is not longer in its infinitive form:
"Write the answer"- "Escribe la respuesta"
"Explain the concept" - "Ilustra el concepto"
On a book/test never says "Ilustrar concepto", "Escribir respuesta"