Levels is a tool in photoshop that you can use to more accurately adjust the tone levels in your images. You can make selections of what you want to change in your image using Paths or you can just change the whole image. To access levels, go to the top toolbar and select Image->Adjustments->Levels… (shortcut CTRL+L for PC or CMD+L for Mac)
This should pop up:
The input levels graph represents the range of tones present in your image. From left to right, the graph goes from brightest tones to darkest tones. The three arrows underneath the graph are how you interact with the tone levels. Dragging the left arrow to the right will start to darken your image, as your input of light tones is darkened. Dragging the right arrow to the left lightens the image in the opposite fashion. The middle arrow represents your mid tones, which will lighten if you drag to the left, and darken as you drag to the right. The mid tone arrow adjusts automatically if you drag the left or right arrows.
You can also change to Red, Green, or Blue levels to adjust those color levels separately by selecting them from the Channel dropdown menu where RGB is displayed. The Output Levels arrows on the very bottom directly alter the entire image’s light and dark levels. I hope this helps!