Term Frequency: Term frequency is a measure of how often a specific keyword appears in a collection of documents. Also known as keyword frequency, term frequency at it's most simple is just the count of how many times a key phrase or word is repeated. For instance, if your keyword is "roof repair" and it's repeated five times, your frequency would be five.
This measure becomes more complicated when tied into tf-idf, which is short for term frequency-inverse document frequency. tf-idf is used to determine how important a word or phrase is to a document in a collection of work based on its search term frequency.
Tf-idf increases in proportion to the number of times a word or phrase appears in a document, but it's offset by the frequency of the word in the general body of language. This keeps words like "the" or "and" from being perceived by search engines and other indexing tools as the main keyword.
Information retrieval and text mining procedures often use tf-df as a weighting factor. Search engines also use a weighting scheme based on a variation of tf-idf to score or rank how relevant a document or webpage is to the query of it's user.
Many of the simplest ranking function simply computes the tf-idf for each query term inputted by the user. Engines like Google use more complex algorithms, but many are variations on the original tf-idf formula.