"I'm weaker than a woman's tear" is an example of what?

"But I am weaker than a woman's tear" is a quote from act 1, scene 1 of one of William Shakespeare's lesser known tragedies Troilus and Cressida and is an example of a simile.
A simile is a literary device in which a comparison is drawn using a connecting word, usually like or as, though in this case that word would be than. A simile can be differentiated from a metaphor in that metaphors often don't use connecting words and similes always do. In this specific simile, Troilus, the speaker and the protagonist of the drama, compares his own strength, or lack thereof, unfavorably to the tear of a woman, a sign of emotional distress traditionally associated with weakness. He uses the literary device to emphasize his point to Pandarus, with whom he is speaking. He is not just weak, he is so weak that even a woman's tear is stronger than he is.
This line is spoken at the very start of the play, after the prologue has introduced the audience to the premise, namely that these events take place in the midst of the Trojan War. Following this prologue, the play proper opens with Troilus, a prince of Troy, explaining that he cannot fight the Greeks because he is too lovesick, hopelessly besotted with Pandarus's niece Cressida. It is this affection that has stripped him of his strength.
The full line of dialogue in which this quote is contained is as follows:

The Greeks are strong and skilful to their strength,Fierce to their skill and to their fierceness valiant;But I am weaker than a woman's tear,Tamer than sleep, fonder than ignorance,Less valiant than the virgin in the nightAnd skilless as unpractised infancy.

As you can see, the line you quoted above is just the first in a long list of similes as Troilus bemoans his own diminished state.