Here is the first line of his work called De Luce:
Formam primam corporalem quam corporeitatem nominant lucem esse arbitror
The first corporeal form which some call corporeity is in my opinion light
He was close and onto something. Unfortunately I parsed through most of his works and he failed to define his key terms and really develop his fundamental notion in terms of physics. In any case he had some neat ideas and these are encapsulated in an Introduction written by a translator of his works named Clare C. Riedl. Here are my notes from her. I highlighted in bold what I thought was important:
For an understanding of the treatise On Light it will be necessary to consider some of the characteristics of Grosseteste's doctrine of matter and form, although his terminology is Aristotelian, the ideas which he expresses in that terminology are often decidedly un-Aristotelian in content. The chief point of divergence is that for Grosseteste matter is not pure potency, as it was for Aristotle, but possesses in its own right a certain minimal reality. Thus Grosseteste speaks of matter as a substance, 'Both corporeity and matter are in themseIves simple substances.';
'It is clear that every higher body, in virtue of the light which proceeds from it, is the form (species) and perfection of the body that comes after it.' In connection with this theory of interaction in virtue of which 'in a sense each thing contains all other things', it is interesting to note the dynamic aspect which Grosseteste assigns to form. Form, that is to say, the first corporeal form, or light, is in his view more than the 'form of corporeity,' the principle of extension, it is also a principle of activity. Every body, he believes, has a motion or activity which is natural to it, because it proceeds from an intrinsic principle. The intrinsic principle from which this motion or activity proceeds must be the form . . .
The subject matter of the treatise is indicated in brief in its opening sentence, where Grosseteste sets forth his thesis that light is 'the first corporeal form.' The remainder of the treatise is occupied with explanation and attempted proof of this proposition together with a detailed analysis of the process by which this first corporeal form united with primordial matter to produce the material universe.
Grosseteste bases his 'light metaphysics' on the consideration of the properties of light and of the nature of material substance. He finds as a characteristic note of corporeity the requirement of extension, 'the extension of matter in three dimensions is a necessary concomitant of corporeity.'
Light furnishes therefore the principle of continuity in nature, for as the first corporeal form it is common to all things in the universe from the lowest of the elements, earth, up to and including even the firmament. Thus 'all things are one by the perfection of one light.' It is also the principle of distinction and multiplicity since the 'things which are many are many through the multiplication of light itself in different degrees.'''
One other interesting I learned from Clare is about the lux/lumen dichotomy in Genesis 1. To quote her:
There seem to be no suitable English words to convey the distinction between lux and lumen. For this reason I have translated both by 'light': indicating parenthetically the Latin word used in each case. The distinction appears to be this: lux is light in its source, whereas lumen is reflected or radiated light. (all above quotes taken from Robert Grosseteste On Light, Introduction Clare C. Riedl)