Avocado extract is a reduced version of the avocado fruit and is available in pill form as a diet supplement. At this point in time, it is still relatively little-known and hard-to-find, but there is a segment of the weight loss business that is trying to promote a supplement as beneficial to a weight loss.
Those claims appear dubious because avocado seems to hardly be of food that enhances weight loss, with its very high fat content and calorie count that goes with that. The one attribute it has that may assist weight loss is its nominal effects on appetite suppression, as foods with a higher fat content tend to satiate the appetite.
There are many health benefits, of course, of avocado such as its high fiber content, essential vitamins and minerals, and monounsaturated fat. The oleic acid helps to reduce bad cholesterol and increase good cholesterol in the bloodstream.
I would recommend, however, that you only take avocado extract supplements if for some reason you are unable to obtain the nutrients naturally. It is always better to get your nutrients from natural food rather than through supplements, and this is no exception.