Annotation services is the kind of data labeling service offered by professional companies to make data like texts, images and videos understandable to machines. Basically, it is the process of making the data recognizable to machines generally through computer vision (in image and video annotations).

And there are multiple types of annotation techniques and methods used to annotate the data with precise labeling. The annotations involve using the tool or software to make the object of interest in an image recognizable to machines to train them using the AI and machine learning based algorithms for predictions in real-life. Source