Stamp Duty Land Tax (commonly just called stamp duty) is a tax imposed on property purchases.
The amount depends on a couple of factors, including:
Property value: the higher the value of the property, the greater the percentage tax. Properties below the threshold don't incur any stamp duty.
First time buyer: if you're a first time buyer, you may have less stamp duty to pay (depending on the property value).
Multiple properties: if you own more than one property (after this one), you may have to pay a surcharge.
More details can be found on the government website, and your solicitor will be able to advise you on this.
Note - even though StrideUp purchases the property jointly with you, it's your home and you are solely responsible for paying the stamp duty.