More information about the tag alt-information
When it comes to objects such as images and embedded video, search engines can only assess the relevance via the alt-information that these objects are given in the html-code: <img src="/images/image.gif" alt="relevant text about the image">.
Even though search engines such as Google ascribe little importance to the alt-information, it is good to provide it anyway: for human visitors, this text is the only reference point if, for whatever reason, images cannot be loaded or they use a special browser such as a web reader.