Subitizing is the ability to recognise numbers of objects or dots by sight rather than by counting. This is a highly important skill for your child to develop and is often underratedl. Your child needs to recognise objects or dots when they are displayed in formal arrays, as in the spots on a dice, or in irregular groupings. Mastery of this skill helps provide your child with a firm mathematical foundation for future learning.
While teachers are aware of the skill, in our experience many of them are not so aware of its importance in your child's maths development. Some children are able to subitize long before they go to school and again, it is a skill that is easily practised on a one to one basis. There are many opportunities in your family's day that lend themselves to practice of this skill. Little and often is better.
In fact it is something that I (Pam) practise constantly with my 3 year old granddaughter, in fact when I hold up my fingers, or show her a number of grapes in my hand, she now looks at me when I say 'how many' with the kind of look only a 3 year old can give, and says 'I know! I know! No counting!'.
Start with a number you know your child can do, it doesn't matter if it's 1. Then move up to the next number only when your child can recognise the one you are working on with 100% accuracy.