Two wild cats live in the same habitat. During winter, there is a scare amount of food, so the cats might compete to get the most food because they want to survive.
Or:
Several plants are growing at the bottom of a forest. Because of the trees above, limited sunlight filters through, so there isn't enough for each plant. Because of this, the plants are competing for sunlight.
Basically, it means organisms in a close relationship being affected by limited resource availability.
Hope this helps!