A board observer attends board meetings and receives board materials but does not vote. The role is common for major-but-not-lead investors, strategic investors who shouldn't have governance rights, and accelerators that want visibility without responsibility.
Observer rights are usually granted to 'major investors' above a holding threshold and can be defined narrowly (board meetings only) or broadly (all committee meetings, all materials). Founders should think carefully about what observers see, especially when those observers represent potentially competitive parties.