A naif is a person with very little experience. When you're just starting a new job, you often feel like a naif.
You might have noticed how similar the noun naif is to the adjective naive. They share a common origin, the French word naïf, which means both "natural, unspoiled, or innocent" and also "foolish." When you describe someone using the adjective form of naif — which can be used interchangeably with naive — you are usually implying that the person is a little childlike or immature.