[linux-yocto] Append vs Expand

[Ran Shalit]

Hello,
I am tring to understand the basic of the language.
Does anyone knows what is the difference between "=" to "+="

I read in free-electrons tutorial:
"
= expand the value when using the variable
+= append (with space)
"
My english is not perfect, so I don't understand the diffrerence
between expand and append.
for example
IMAGE_INSTALL = "dropbear" means that it is added to other packages ?

Regards,
Ran