What the speaker is talking about is the idea that all people share the same troubles and that all people's troubles come to nothing in the end.
The speaker is imagining the feelings of a Roman who was, centuries ago, in the same place that he is now. He imagines that the Roman had the same kinds of concerns that he has and he says that the tree of human life is "never quiet." This shows us that people have always had problems.
But then, at the end of the poem, he also says that the Roman is dead and buried now. This implies (whether for good or bad) that the speaker, too, will one day die and will no longer have to think of his problems.
The speaker concentrates on heavy winds (gales) that blow through Wenlock Edge, a region in western England. The geographical and meteorological conditions have not changed over the centuries, though the political situation has altered. At present, winds of hurricane velocity threaten the existence of the forests there. It was also this way in the past, during the time when the Romans occupied England in the first five centuries c.e. The dangers of volcanic eruptions (line 2) are not diminished. The threat to young trees (‘saplings”) is the same as in the past. In line 17, Housman repeats line 3, “The gale, it plies the saplings double” to stress the fact that human beings have everywhere and always been subject to the sorts of stress and trouble symbolized by the heavy wind. In terms of personal trouble and thoughts that hurt, the Roman is thus the emotional forebear of modern human beings. The land is the same, people are the same, and mental anguish is the same.