have your word

What does 'have your word' mean?

“How much do you want? I’ve got gold.”
“Not gold,” said Griphook. “I have gold.” His black eyes glittered; there were no whites to his eyes.
“I want the sword. The sword of Godric Gryffindor.”
Harry’s spirits plummeted. “You can’t have that,” he said. “I’m sorry.”

.......................

Back in the smallest bedroom, Harry made the offer, careful to phrase it so as not to give any definite time for the handover of the sword. Hermione frowned at the floor while he was speaking; he felt irritated at her, afraid that she might give the game away. However, Griphook had eyes for nobody but Harry. “I have your word, Harry Potter, that you will give me the sword of Gryffindor if I help you?” “Yes,” said Harry. “Then shake,” said the goblin, holding out his hand. Harry took it and shook.