SEO stands for search engine optimisation.
An SEO agency generally focuses on improving your website's visibility in search engines such as Google. Depending on the agency, this can include keyword research, website improvements, content creation, technical SEO, local search optimisation and link building.
The aim is to help more people discover your business when searching for relevant services. For example, if you're a plumber in Melbourne, an SEO agency might work on improving your visibility for searches related to plumbing services in your area.
SEO can be valuable, particularly if you're competing in a busy market.
However, results can take time, and improving your rankings doesn't guarantee that visitors will contact you or become paying customers.