A Null subject language is a language whose grammar allows sentences that do not explicitely include a subject. Often, the grammatical person of the subject can be understood from the verb endings.
Examples of these language include some Romance languages (such as Spanish, Italian, Romanian, etc), Arabic, and several Asian languages, especially Japanese.
Other languages (sometimes called non-null subject languages), require each sentence to include a subject -- this is the case for most Germanic languages, such as English and German.