I've heard the word stretching a lot when my ...

This are the stages of a cannabis growth: SEEDLING: From seed until the cotyledons die (the round baby leaves that first open when a seed sprouts), this usually takes 3 weeks VEGETATIVE: With an established root system, the plant grows fast. For photoperiods this stage lasts as long as you keep the light schedule above 12 hours a day. Autoflowers skip this stage. When the day light (outdoors) or the artificial light is less than 12 hours a day flowering stage begins, which comprises of 2 stages: PREFLOWER / STRETCH. the plant enters a rapid growth stage, its height doubles or even triples rapidly, making space for the flowers. At the end of this stage there are first signs of flowers: FLOWERING: the plant stop growing and focus on developing buds

Stretching is vertical growth. Ex: the plant was 40cm before stretching, but after stretching she became 120cm tall. During the late veg and early/mid flower almost all the way to the end, depends on genetics.