Tax point
In one sentenceThe date on which VAT on a supply becomes due, determining which tax period's return the tax goes into.
Simply put
VAT becomes due at the earliest of three events: delivery of the goods or completion of the service, issue of the invoice, or receipt of all or part of the price.
That is why the VAT on an advance payment you receive before delivery is due in the period of receipt, not in the period of delivery.
Example
A furniture factory received an advance payment in March for tables it will deliver in May: the VAT on the advance goes into the March return, and the VAT on the rest goes into the period in which delivery takes place or the invoice is issued.
Don't confuse it with
Tax periodThe period is the month or quarter combined in one return, and the date of supply determines which period each transaction goes to.