Grease the Groove (GTG) refers to doing several small sets of a movement throughout the day, with the intent of completing a large number of repetitions spread throughout the day rather than devoting a block of time to exercise.
This type of training is popular with kettlebell and calisthenics enthusiasts.
An example of GTG would be installing a pullup bar in your house and whenever you walk by the pullup bar, you do a set then continue on with your day. Over the course of the day, you may end up walking by the bar 15 times, getting in 15 sets without having to block out time to exercise, change into exercise clothes, or get all sweaty (!!!).
Another example would be getting in a set of calisthenics every hour while you’re awake.
This method is good for developing skills and strength with movements because you can do the set in a fresh state and be more mindful of maintaining form for the movement.