When we create a variable and don't initialize it, then some (random) number called garbage value is assigned to it.
The garbage value is not assigned, rather the value is already there. When you allocate a variable you are reserving a piece of memory - until you overwrite it that memory will contain whatever "random" information was there before.
As a metaphor, think of allocating a variable like buying a piece of land - until you do something with it (like build a house) the land will just have whatever trash was already sitting there (like an old crumbling house).
Some languages will automatically fill newly allocated variables with zeros - this takes time to do. In more "do-it-yourself" languages like C this extra behavoir is not guarenteed (though on some systems memory is cleared regardless of language, for example as a security measure)