The Annual Investment Allowance, or AIA, lets a business deduct the full cost of most qualifying plant and machinery from its taxable profits in the year of purchase, rather than spreading the relief over several years. There is a cap on how much you can claim in a period, set by government and published on gov.uk, and some assets are excluded.
The practical effect is timing. Without AIA you would relieve the cost slowly through writing-down allowances; with it, qualifying spend comes off in one hit, which can pull a tax saving forward into the year you actually bought the kit. The total relief over the life of the asset is broadly the same either way — AIA mostly changes when you get it, and the value of that relief depends on your prevailing tax rate.
A few things to watch. Cars are generally excluded. The limit applies to the business or group as a whole, not per asset, and it is proportioned if your accounting period is longer or shorter than twelve months. Connected companies and groups usually have to share a single allowance rather than each claiming one. Check the current cap on gov.uk before you plan spending around it.
It also matters how you fund the purchase. Hire purchase and lease purchase normally let you claim, because you are treated as buying the asset; most leases do not, because you are not. That is worth raising with your accountant before you pick an agreement type, not after.
This is general information, not tax advice. Confirm your own position with your accountant or HMRC.