We use the term gelling to describe the feeling/understanding, that the activation process is building to make sense for us.
Because activation is done in layers at first it our understand/comprehension doesn't seem solid or sound. With each activation layer it becomes more solid or harmonious to our conscious mind. At first our comprehension is like it's in liquid form... you feel like you have nothing to hold. As it 'gel' it becomes more possible 'hold' onto the information.
For you it might make more sense if we described it as tuning a musical insturment. At first it sounds flat, with the tuning process (activation layers) the sound is pulled to the correct resonance. Until it sound harmonious to you.
It's just a way of describing the journey from vague comprehension of the text to sound comprehension of the text. Once it's sound right to you it has 'gelled' 
Alex
