A preset is saved settings made within Lightroom. 

In develop mode you can make adjustments to an image including variables such as: exposure, contrast, tone curve, lens profile correction, etc. The possibilities are endless. 

Once you have made all the adjustments you would like you can save this by clicking the small "+" icon to the right of the Presets heading in the presets panel.

A menu will appear for you to select or deselect what you would like to be saved within your preset:

Hit create and you've officially created a preset! To then use the preset just click on it within the folder you saved it under 'Presets' and it will apply to the image. You can then sync the preset to other images or leave on just one. 

You can also purchase Third Party presets where all the work has already been done! Some popular ones are: VSCO, Mastin Labs, and Red Leaf. They will provide you with install instructions.