What are joined and unjoined fonts?
A 'joined' font may refer to a type of script typeface, where all characters of a word are connected by single lines. Most so-called 'handwriting' fonts make use of these glyphs. And some fonts, such as your example, may offer an additional isolated glyph version, like this font: 



As the name 'script' implies, it is derived from handwriting, and can still generally be seen in writing exercises in primary schools.



Alternatively, those terms might be referring to fonts that offer versions with and without ligatures, the usually curvy lines that connect certain sets of characters:




  In writing and typography, a ligature occurs where two or more
  graphemes or letters are joined as a single glyph.
  source


Like script typeface, ligatures derive from handwriting.
A famous common example is the ampersand &amp;.



Usage

Whenever you come across a font with different character sets, they are offered as separate fonts (under the same family name, sometimes archived together in .rar, .zip, or .7z files, which have to be unpacked first).
You install the fonts, and they will show up in your editing programs as separate fonts (e.g., using the example above, the fonts will show up like "Debbie Hepplewhite Print" and "Debbie Hepplewhite Joinit").



If you have any other questions, let me know, and I will update my answer.