During the Appstore approval process Apple were concerned that it would not be clear that some features of Texpad were only available if the user has installed a LaTeX distribution. We altered the iTunes description and included a download link in Texpad itself, but just in case you have downloaded it without realising, and found your way here I will explain what LaTeX is, and how to get it for free.
LaTeX is a typesetting system. It is often described as a Word Processor, though this is misleading. Both pieces of software share the aim of allowing a user to create beautiful documents, but that is where the similarity ends. Word Processors present the user with a What-You-See-Is-What-You-Get where the user can alter text attributes, such as font and colour, via menus.
In LaTeX documents are created as plain text files with LaTeX markup commands to denote structure and formatting, then the LaTeX source is typeset to PDF files by the LaTeX executable. The resulting document is of far higher quality, but the LaTeX source is a lot less intuitive than the WYSIWYG interface you expect from a Word Processor. This is where LaTeX editors such as Texpad come in. They provide an interface especially suited to writing LaTeX documents, and handle the interaction with the LaTeX typesetting system for the user.
The LaTeX typesetting system is a very large piece of open source software, available here. Due to its size (2GB), and many users’ preference for alternate distributions we did not include it in the executable, but if you follow that link you can download and install it for free. When it is installed, reboot Texpad and follow a LaTeX guide (such as this) to get started.
As you are new users, two pointers: LaTeX is pronounced lay-tech, and yes it really does have that absurd capitalisation.