though my Soul more bent To serve therewith my Maker(lines 4-5)
The speaker claims that the desire for service is built into his constitution. His "Maker" is like a magnet that attracts his soul.
"Doth God exact day-labour, light denied?" (line 9)
The speaker questions God's principles. Would God ask for the impossible? What would be the point of that? Think of "day-labour" as being like physical work that would require light.
But patience, to prevent That murmur, soon replies, "God doth not need Either man's work or his own gifts; (lines 8-10)
Patience says that the speaker has God's principles all wrong. What this really means is that the speaker is correcting himself, because "patience" is one of his principles. All this virtue stuff can get confusing.
who best Bear his mild yoke, they serve him best.(lines 10-11)
The supreme principle of patience is that you have to endure difficulty and not question the hand that fate deals you. If you want to go left and fate decides you should go right, well, you're going right. A "yoke" is an instrument to bring people under submission, in this case to divine justice.
They also serve who only stand and wait." (line 14)
Not much to say here. This line pretty much defines "patience."