Paint is created by combining dry pigment and liquid. In oil paints, the liquid used is linseed oil. You purchase oil paint in tubes. The paint is thick. So when you squeeze it out onto your palette, you mix it with your palette knife and use a stiff brush to apply it to your canvas. The thickness and composition of the paint means that it is very slow drying. Paintings done in oil typically take around 72 hours to dry.
The upside of the delayed drying time is that you can make adjustments to refine your painting. This can be done for up to twelve hours after you paint the canvas. You can make gradual shifts from one color to another. If you are using oil paints, you can also completely remove things from your canvas with your palette knife or a wet rag. Once the paint has dried completely, you can then put new colors on top of the original layer of paints. However, be aware that in order to avoid cracking, each subsequent layer must be thicker than the previous