Answer:
Among other vows, nuns were dedicated to lives of poverty - meaning they could have no money or possessions of their own. It was intended that everything needed in a nunnery would be provided by the nun's own work (making things to use, growing things to eat and so on), but the reality was that some things had to be bought.
In order to make money many nuns became proficient at needlework: sewing, embroidery and needlepoint. Their finished products could be sold and the money went into the coffers of the Prioress or Abbess in charge. This money could then be used to buy cloth, sewing thread and anything else that was needed. The money belonged to the community, not to an individual nun, so the vow of poverty was not broken. Nuns also took in laundry, or kept bees and sold the honey and beeswax. Chapter 57 of The Rule of St Benedict allows for craftspeople and artists in monasteries and nunneries to produce items for sale, "but let it always be given a little cheaper than it can be given by seculars".
Nunneries also earned income from land and assets they owned. For example, the prioress of Dartford nunnery owned extensive tracts of land, woodland, grazing marsh, chalk quarries, several mills, tenements and inns in North Kent and throughout the county of Kent. Additional properties, lands and church advowsons (*see note below) were held in Bedfordshire, Buckinghamshire, Dorset, Glamorgan, Herefordshire, Hertfordshire, the City of London, Norfolk, Suffolk, Surrey and Wiltshire. These lands and properties generated a considerable income.
The nunnery of Nun-Appleton in Yorkshire owned lands and property generating over £30 of income per year in the 13th century - a huge sum at that time.
So although individual nuns were very poor and had no money of their own, the Church as an institution was extremely wealthy and held huge amounts of land and property.
* An advowson was the right to appoint a priest, chaplain, vicar or other clergyman to a position in a rural or urban church or chapel.