Growth Stages allow you to set different meshes the plant uses depending on the current growth of the plant.
You can find these settings in the plant blueprint like for example the BP_TomatoPlant then select the BP_PlantComponent and in the details panel in the Plant Settings category find the Growth category in that category is the Growth Stages setting like shown in the image below.
You can enable or disable this system using the Use Growth Stages setting.
Here you can set the different static or skeletal mesh the plant will use at certain growth amount. To change when the meshes will be used you can set the Growth setting for that Index in the Growth Stages setting. Here you can also change the static mesh or skeletal mesh that will be used for that growth value.
You can also set the Effects for each growth stage this can be a sound and/or particle effect that will be played at the plants location.