Search Engine Optimisation (SEO) is usually done on a website by a digital marketing agency london to affect the “organic” ranking position of a company. The reason companies want their websites to rank highly is so they get more visitors. Generally the top positions get the most visitors as they are more relevant to what the user is searching therefore companies pay for digital marketing services.
There are usually two forms of SEO, white and black hat. Black hat SEO is a method that is disapproved by the major search engines, this can include keyword stuffing (using the same keyword over and over again, so that you rank highly for that particular keyword) and mass low quality link building in a short period of time. White hat SEO is approved by the search engines, these are general methods that are beneficial to the users experience, so if your site is relevant to ‘hats’ then you should use that appropriately in your title and content, backlinks should also be very natural and high quality, they should not be mass build. Wikipedia has more information on SEO.