I have liv'd long enough: my way of life
Is fall'n into the sear, the yellow leaf;
And that which should accompany old age,
As honour, love, obedience, troops of friends,
I must not look to have; but, in their stead,
Curses, not loud but deep, mouth-honour, breath,
Which the poor heart would fain deny, and dare not.
This isn't really an "ask historians" question as much as "ask a literature professor" but I'll answer anyway.
Setting aside the poetic verse, the first two lines say: "I have lived long enough: my way of life is fallen into the sear, the yellow leaf"
"I have lived long enough" is simple enoigh to understand. It means "I'm not young."
"My way of life has fallen into the sear" is a little more complicated. What is "the sear?" A simple google search of "Sear definition" brings up a dictionary entry. Most of the way down, an "archaic" meaning is "to wither." This line could mean "My way of life has withered." Given the context, the speaker having said he's not young, this meaning seems reasonable.
"The yellow leaf" in THIS context seems to be a metaphor for "my way of life." It's yellow, not because it comes from an ancient special tree, but because it has withered, as leaves do in the autumn, much like "my way of life."