The terms of an addition are called addends.
In the sum: $$2+42+9$$, the addends are: $$2, 42$$ and $$9$$.
The sum of natural numbers fulfills two basic properties:

Commutative property $$a+b=b+a$$
When we do a sum, we can change the order of the the addends and the result will be the same
$$$4+6=6+4$$$ $$$2+42+9= 42+2+9=9+2+42$$$

Associative property $$(a+b)+c=a+(b+c)$$
When you add up three or more numbers at once, we can add them up in pairs and finally add up the results.
In the addition $$15+2+7$$
can be calculated first $$15 + 2 = 17$$ and then $$17+7 = 24$$.
Or if you prefer, you can start adding up the last numbers: $$2+7=9$$, and then $$15+9 = 24$$.
 In mathematics, brackets are used to show which operation is done first (in this case, the sum).
$$(15+2)+7$$ means that firstly we do $$15+2=17$$ and then $$17+7 = 24$$.
Looking at this example, the associative property can be applied as follows: $$$(15+2)+7=15+(2+7)$$$

Neutral element $$a+0=0+a=a$$
The $$0$$ is the neutral element of addition, since any natural number added to $$0$$ results in the same number.
$$$9+0=9$$$ $$$0+4=4$$$