Mediums are used to adapt the consistency, drying time and finish of your painting.
You can achieve a variety of different sheens and either extend or shorten the drying time of the paint, depending on which you use.
Mix them directly with the paint on your palette, or dip your brush into them as you would with water.
Most oil painting mediums are tinted slightly yellow colour, you need to be cautious when mixing them with lighter colours. Over time they tinge your paintings yellow, so just be mindful of how much you use when mixing with paint.