The Ending was an important event in the Glade. It began when Teresa entered the Glade, included the removal of the Glade's sun, and caused the doors of the Maze to remain open at night, meaning that nothing was stopping the Grievers from infiltrating the Glade. This was also the point at which the Creators stopped giving weekly supplies to the subjects, thereby forcing them to venture outside the protection of the Glade. It would also presumably stop the sending of Greenies, as Teresa had a note saying she would be the last one, ever. However, since they didn't stay in the Glade for a month after Teresa came, it is impossible to know for sure
Teresa's arrival in the Glade marked the beginning of the Ending. When she woke up after several days in a coma, the artificial sun disappeared and left blank grey sky that, according to Chuck, resembled a ceiling (in The Fever Code, it was confirmed that this was indeed a ceiling, and the sun had been holo-tech). In consequence after a few days, the plants began to wilt. However, there was still a dim light. The weekly supplies were cut, thus limiting the available food. Also, the doors stayed open all night, so the Grievers could come and strike the Gladers – in the book, they came several nights, each time taking one Glader; in the film, they raided the Glade only once but killed almost half of the Gladers.
This made most of the Gladers suspicious of Teresa, most notably Alby, who locked her in the Slammer. Thomas thought Teresa innocent, and Newt told him, "Because she came and everything is suddenly do-or-die?", to which Thomas replied, "She's just a pawn – they sent her here as our last tool or hint or whatever to help us out."