Three short lines, and almost nothing happens: cows are grazing, and then summer rain arrives. What makes the poem work is the phrase "even more content." The cows were already calm, eating their grass, and the rain doesn't disturb them. It deepens their ease. The poem seems to suggest that the rain is a small extra pleasure rather than an interruption.
The speaker never says "I feel," but the feeling comes through in what they choose to notice. They watch the animals closely and read contentment into them, so the speaker is probably calm too, perhaps sharing in that quiet moment. There's also a gentle contrast hidden here. People often hurry indoors when it rains, while the cows simply carry on. The mood is peaceful, unhurried and a little humorous in its simplicity. Reading it, you can almost hear the soft patter of rain and feel the slow rhythm of animals who want nothing more than what they already have.