A paragraph before this one would help.  Something like:

Translating strings for a website like edX is more complicated than simply translating sentences from one language to another.  Strings in programs sometimes need to have data inserted into them before being displayed to the user.  Data placeholders label the places in the string where the data will go.  Strings can also have markup like HTML included.  It's very important to preserve the placeholders and markup so that the web site will work properly.