A player is Assicoation Grown in the Football Association that they developed as a youngster. Similarly, they are Home Grown at the club where they spent time during their youth career. Some competitions require squads to be submitted with a minimum number of association grown and home grown players, for example, the European Champion's Cup.
Association Grown[]
A player receives Association Grown status when he has trained at any club in the country he is currently playing in for 3 years and can only be achieved by a player up to 21 years of age. Nationality is not a requirement of Association Grown status. A player can only ever receive Association Grown status at one country. In practice a 18 year old Spanish player can play for Aston Villa for two years then move to Arsenal, play for one year and receive Association Grown status for England.
Home Grown[]
A player receives Home Grown status when he has trained at the same club for 3 years and can only be obtained by a player up to 21 years of age. He will also receive home grown status, as he has trained in the same country by training with the same club. In practice, this means that a French 16 year old can who plays for Arsenal until he is 19 will earn English Association Grown status and Arsenal Home Grown status.