Marcus and his friend see darkening clouds, and they feel drops of rain.
A conflict is a struggle between to opposing forces. In this sentence, Marcus and his friend are noticing the incoming storm. This is a character vs. nature conflict because Marcus and his friend will have to struggle through the storm.
There are 4 different types of conflicts in literature. Character vs. character, character vs. society and character vs. nature are all external conflicts. The fourth, character vs. self, is an internal conflict.