Rather than give you the literal definition of algorithms I will explain there definition in relation to search engines.
Search engines tend to use software to review and include web sites.
Generally because it would cost too much and take too long using humans to review sites. Algorithms are basically the rules that have been programmed into their software to review your web site. Each search engines has different algorithms which they change on a regular basis. These algorithms are kept secret however it is possible to work out some of the algorithms. You perform searches on the search engines looking at the key phrases you are interested in.