Spam typically refers to unsolicited bulk or mass-mailing messages in e-mail, or inappropriate off-topic or flooded messages in chat fora.
Since most online resources revolve around a certain theme or community, outsiders who post messages about entirely unrelated subjects (especially when those subjects are advertisements or money-making schemes, such as phishing or multilevel marketing schemes) are considered spam. However, even on-topic messages can be considered spam if the are excessively repeated or dethreaded.
Many resources have specific policies against spam. On BoyChat, for example, it is prohibited by the guidelines.