In just four lines, the speaker watches a heavy-set man in a grove early in the day and sees a bear in him. The poem's idea is a quiet one: the man seems to carry himself the way a bear might, with a slow, careful way of settling down. "His care in sitting" is the detail that makes the comparison work. Big bodies move gently when they want to avoid fuss.
The last line turns back to the speaker. "My hide of a good face" is an odd, slightly awkward phrase, and that awkwardness is part of the point. The speaker seems to be wearing a pleasant, friendly face like an animal's skin, perhaps as a way of being polite or staying unnoticed while watching. Judging by the wording, there may be a hint of self-awareness here: the speaker is also playing a role.
The mood is calm, observant and a little amused. Nothing dramatic happens, but the morning stillness makes the human and animal worlds feel close together.