Ere half my days, in this dark world and wide, (line 2)
The poem does not start out on optimistic footing. The world is "wide" and full of possibility, but its "darkness" means that there is no way to distinguish between good and bad. The speaker is left to grope through this darkness without either his literal vision or the metaphorical vision of truth.
And that one Talent which is death to hide Lodged with me useless, (lines 3-4)
The talent has not gone away – it is merely hidden or "buried" out of reach. It cannot be used, but its potential remains. The speaker worries that he will suffer a metaphorical "death" because of a condition he cannot control.
lest he returning chide (line 6)
In the "Parable of the Talents," a lord scolds his third servant after returning from a trip, and in this poem, the speaker believes that the lord is a stand-in for God. What trip would God be returning from? Maybe the speaker means that life is a brief separation from God, who will "return" after one's death.
who best Bear his mild yoke, they serve him best. (lines 10-11)
According to "patience," we all wear a "yoke" to prevent us from straying off course. The yoke sometimes prevents us from doing the things we want or could be doing, but in the end, the farmer knows best, right?
They also serve who only stand and wait." (line 14)
Nobody likes to wait. At the dentist's office, at the train station, in line at the movies: waiting is rarely fun. But if you're waiting on someone, that's different, especially if that someone is a king. You have to be ready at the drop of a hat to serve the Big Guy.