"Purnima" means "full", and refers to the "full" moon. Every full moon or Purnima is a celebration of the Spiritual Teacher. But the full moon in the Hindu month of Ashad (which usually runs from sometime in June to sometime in July) is called "Guru Purnima", and is generally considered to be the brightest full moon of the year in the Hindu tradition. In the yearly cycle, it is the day set apart for remembering the Guru or Divine Teacher, and paying homage to, and worshipping His or Her appearance in human Form.
Read a full text about Guru Purnima written from Sri Samana: https://www.flameofsilence.org/writings/gurupurnima