State Aid is defined on the Gov.UK website as “any advantage granted by public authorities through state resources on a selective basis to any organisations that could potentially distort competition and trade in the European Union”. This broadly means any business or training support or grants, that have been funded by the UK government or the European Union, that are designed to help improve a businesses’ position in the market.
State Aid is not allowed in the EU because a business can receive support that give it an advantage over its competitors. Under the De Minimis regulation, however, businesses could receive State Aid up to the value of €200,000 in any rolling 3-year period without the need to formally notify the European Commission. If you are part of a group of companies, the €200,000 limit applies to all companies under the control of the parent company - so you will need to add all the State Aid received by each company in the group together, to get the total amount of State Aid received.
You can find more information on State Aid at https://www.gov.uk/guidance/state-aid