The word untrammeled means not restricted or deprived of freedom. From the phrase, it means the wilderness is free from the habitations of man. That is, humans don't live in the desert and as such, the wilderness has its freedom of humans.
Explanation:
From the phrase, "A wilderness, in contrast with those areas where man and his works dominate a landscape, is hereby recognized as an area where the earth and its community of life are untrammeled by man…", it is talking about how the wilderness is one of the few places on earth that is not inhabited by man and therefore free from man and his effect on the environment.
From the phrase, the speaker is trying to convey that the wilderness is unlike other places that is dominated by man and his activities.
