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The concept of addition comes from combining two "groups" of objects. For example, putting 2 apples and 3 apples together, and count the number of these apples afterward, the answer becomes 5.
To describe this, we use the notation 2+3 = 5, read "two and three is five", or "two plus three is five". The method of performing addition between two numbers is just do this on apples. However, we now hope to have a systematic method for this. We have to do more observation on the example above: To put 2 apples and 3 apples together, one may follow the following steps:
(a) To move one of the three apples to the group of two apples, and the group becomes 3 apples.
(b) To move one more apple to the group, then this group becomes 4 apples.
(c) To move the last apple to the group, then the group finally has 5 apples.
From this process, we find that the number of apples in the chosen group (the group with 2 apples originally) changes from 2, to 3,4, and 5. This tells us that, to get what 2+3 is, one needs to "count to the next number" from 2 for 3 times. This is similar for various numbers.