A holding deposit is a fee of one week's rent you must pay to secure the property during the offer process. Once tenancy begins, this deposit will be credited towards your first month's rent.
Once an offer is accepted by both parties, a holding deposit is required to secure the property and move forward in the offer process. Once paid, the listing will be taken off the market and reserved for you. You'll then move onto the next step in the offer process, referencing.
If the deposit is not paid within 24 hours, the property is up for grabs, meaning the owner may accept offers from other applicants and you will risk losing the tenancy. To avoid this, send your offer with the deposit pre-authorised in advance.