A pledge is basically very serious formal promise. You can pledge allegiance to your country, you can pledge to keep a secret, and you can pledge a sum of money to a cause.
Pledge can be used as both a noun and a verb. As a noun, it can be a solemn promise you've made. Or even the person who makes that promise, like the freshman pledges who take an oath to join a fraternity in college. As a verb, it describes the act of promising. “I pledge you, I pledge myself, to a new deal for the American people,” vowed Franklin D. Roosevelt, the 32nd President of the United States.