Old Mr. Wickham was the elder Mr. Darcy's steward. A steward is one who manages property, financial, and sometimes personal affairs.
Here is the younger Mr. Wickham explaining what his father had done, in Vol. 1, Ch. 16:
My father began life in the profession which your uncle, Mr. Philips, appears to do so much credit to -- but he gave up every thing to be of use to the late Mr. Darcy, and devoted all his time to the care of the Pemberley property. He was most highly esteemed by Mr. Darcy, a most intimate, confidential friend. Mr. Darcy often acknowledged. himself to be under the greatest obligations to my father's active superintendence, and when immediately before my father's death, Mr. Darcy gave him a voluntary promise of providing for me, I am convinced that he felt it to be as much a debt of gratitude to him, as of affection to myself.''