Surrender in war means a person or group must give up control of something of value to another person or group. The terms are the promises each side makes to the other so the conflict is ended.
For example, in 1865 at Appomattox, Virginia, some of the terms of surrender included allowing soldiers to return home as long as they gave up all their military supplies. They could keep their horses or mules if they owned them. The Union would also provide them with food. These are examples of terms that could be offered at the end of a battle even though these specific terms were unusual. A more common practice was to only allow the losers to collect and bury the dead.
When Confederate General Simon Buckner sent the surrender letter to the Union forces, he asked for the terms of surrender. General Ulysses S. Grant did not offer such terms at Fort Donelson. Instead, he wanted “unconditional surrender,” meaning there would be no bargaining. By having no terms offered to them, the Confederates had to give up or continue to fight. They agreed to surrender without terms, winning an important victory for the Union. It was this event that earned Grant the nickname of “Unconditional Surrender” Grant, a new meaning for his initials U.S. Grant.