If you meet certain criteria, then you may need to register and account for VAT (Value Added Tax) on your sales and purchases.
You will need to charge and collect VAT on your sales, but may also be able to claim back VAT on purchases that you have made, the net being paid over to HMRC.
This may be further complicated if your buy or sell goods and services outside the UK, and again if outside the EU.
We can advise you on these things, assisting with everything from initial registration, what scheme you should be on, accounting for VAT, and filing your VAT returns with HMRC.