Oft did the harvest to their sickle yield, Their furrow oft the stubborn glebe has broke; How jocund did they drive their team afield! How bow'd the woods beneath their sturdy stroke!
Now the speaker imagines the kinds of things these guys did back when they were still alive.
These are country folks, remember (since they were described as "rude," and since we know from the title that this is a "country churchyard"), so they were farmers.
They often harvested their crops with their sickles (a sickle is a curved knife, like this).
More farmer lingo in this line: the "furrow" is a long, narrow, shallow hole that you drop seeds into. "Glebe" is an archaic word for farmland. Farmers would cut the furrow into the glebe using a plough, but if the ground is really hard to break into, you might describe it as "stubborn." Here's a pic of a plough cutting a furrow.
The speaker imagines that the farmers were cheerful, or jocund, as they drove their teams of oxen or mules into the field to plough.
The woods bowed to the stroke of their axes as they cleared forests to make their farms.
More personification! Even if you're really handy with an axe, the trees aren't going to bow down to you out of respect. They're just going to fall over.