A long-tail keyword is a longer, more specific search phrase, such as "invoicing app for freelance designers", that few people search but that says exactly what they want.
Each one has little traffic, but there are a great many of them, and the big sites that own the short phrases rarely have a page for each.
Because the phrase is specific, the person searching it is close to choosing something. A page that matches it exactly tends to convert well.
Why it matters for a launch
A new site cannot rank for "invoicing software". It can rank for the specific phrase that describes what it does, and that is where a maker's first search visitors come from.
Read more: Why a new website isn't on Google yet