Most probably it is random letters/ words to try to fool spam filter.
A lot of spam filter works by statistical analysis of the content. Roughly, they computed the 'probability' of every word to be spam and if the total for the message is higher than the limit, it is considered spam.
A message containing a word like 'sex' specially in accordance with 'free', 'big',.. has a great probability to be spam.
But a message of ten pages having only one occurence of the word 'sex' and associated with words like 'species', 'monkey' is probably genuine.
So they add words / letters, to try to lower the statistical analysis of the rest of the message, but as it is not part of the message they want you to read, they put it in very tiny fonts or try to put in the same color as the background, if it is html.