This agent selects actions based on the agents current perception or the world and not based on past perceptions.
For example if a mars lander found a rock in a specific place it needed to collect then it would collect it, if it was a simple reflex agent then if it found the same rock in a different place it would still pick it up as it doesn't take into account that it already picked it up.
This is useful for when a quick automated response is needed, humans have a very similar reaction to fire for example, our brain pulls our hand away without thinking about any possibility that there could be danger in the path of your arm. We call these reflex actions.
This kind of connection where only one possibility is acted upon is called a condition-action rule, written as:
if hand is in fire then pull away hand
The simple reflex agent has a library of such rules so that if a certain situation should arise and it is in the set of Condition-action rules the agent will know how to react with minimal reasoning.
These agents are simple to work with but have very limited intelligence, such as picking up 2 rock samples.