An oblique pyramid is one where the apex is not over the center of the base. The opposite is called a 'right pyramid'. In the figure above, drag the apex point and note how the pyramid "tilts" to one side when the apex is not over the base center point.
In an oblique pyramid, the usual ways to calculate its surface area no longer work. You would have to break the shape down into parts, calculate them separately, then add the results together. However the method for calculating volume works for both right and oblique pyramids. See Volume of a pyramid.
If the base is an irregular polygon, then the base usually has no defined center point*, and so the distinction between right and oblique makes no sense.
* We say 'usually' because a few irregular polygons do have a center. For example, the rectangle has a center, - a point that is equidistant from each vertex.