The distributive property is a way to expand multiplication across addition. You can think of it as a way of "getting rid" of parenthesis. It works the following way

a × (b + c) = a × b + a × c.

also written as

a(b + c) = ab + ac

Basically, instead of the a multiplying the whole (b + c), it multiplies the individual b and c themselves.

To see how it works, imagine a common mulitple you know, like 5 times 6

5 × 6 = 30

Now think about what numbers add up to 6. 1 + 5 = 6 for instance. So we could rewrite 5 times 6 as

5 × (1 + 5)

According to the distributive property though,

5 × (1 + 5) = 5 × 1 + 5 × 5 = 5 + 25 = 30

The coolest part about this is that it works no matter what you add together to get 6. So just by using this property you can see the following

5 × 6 = 5 × (2 + 4) = 5 × 2 + 5 × 4 = 10 + 20 = 30

5 × 6 = 5 × (3 + 3) = 5 × 3 + 5 × 3 = 15 + 15 = 30

5 × 6 = 5 × (1 + 2 + 3) = 5 × 1 + 5 × 2 + 5 × 3 = 5 + 10 + 15 = 30

If you're still confused I encourage you to try this when any two numbers and try to ask yourself what is happenning when multiplication is distributed over addition.

You're example is a little unusal since most distribution problems occur with a parethesis. However distribution can happen in reverse too, something more commonly though of as factoring. In order to help you out, think of the problem like this.

DISTRIBUTIVE PROPERTY:

a × (b + c) = a × b + a × c.

PROBLEM:

[ ] = 15w + (-w) + 5

And figure out what should be in the blank.

HINT: It may be easier to ignore the 5 for a while.

Try it yourself. The solution will be at the bottom of this post.

SOLUTION:

15w + (-w) + 5 = w(15 + -1) +5

Here the distribution is shown in reverse. 15w + (-w) could have come from a distribution of w(15 + -1). The 5 has no part to play in this really and is just hanging on at the end.

w(15 + -1) +5 = w(14) + 5 = 14w + 5

The eventual answer is 14w + 5 after making the incredibly challenging and technical arithmetic computation of 15 -1 = 14 :D Don't forget that adding a negative number is like subtracting it a positive one.