Lexical roots are the smallest possible parts of words that, when combined appropriately, form functional words. Lexical roots differ from grammatical roots in that they convey things, actions, or attributes, while the latter convey grammatical functions such as tense, person, number, and mood, among other things. While the distinction is not always clear semantically, it is morphologically. Lexical roots are all the word parts, also called morphemes, that form the stem of a verb or noun prior to its inflections being added. Particles being the exception, these are fully formed lexically and do not require inflections.
Speakers of Old Cree would have relied heavily on the combinatorial properties of lexical roots to form words and this is no different for speakers of contemporary dialects. This feature of Cree is described as polysynthesis. A polysynthetic language is characterized by words consisting of several morphemes. In such a language, a single word may function as a whole sentence. The possible combinations are limitless. While we certainly have a sense of some of the words that speakers of Old Cree would have likely spoken, an accurate reconstruction of the proto-language must limit itself for the most part to lexical and grammatical roots.