Marian apparitions are known physical appearances of the Virgin Mary on Earth. These appearances are often linked with a type of message Mary wants to communicate. There is always at least one witness to an apparition, and it is not uncommon for the witness to later become a saint or holy figure within the Church.
Mary wants nothing more than to lead us to her son. This is why she is such a powerful intercessor. Marian apparitions is another way of Mary leading us to Jesus.