a stiff back

"Stiff" is the opposite of "flexible". It describes when something doesn't bend easily. "Stiff" can be used to describe body parts that aren't flexible because of injury, posture, or not enough exercise. Some body parts that often become stiff are:

• back
• shoulders
• legs
• knees
• neck

(something) is getting (adjective)

"Getting __" means "becoming ___".

You use this phrase to talk about a process that started in the past, has a certain goal or end point, but hasn't finished yet.

For example, when you sit on the ground for a long time, your back will eventually get very stiff. When you ask someone:

Is your back getting stiff?

...you're asking whether they are starting to have a stiff back. You're assuming that their back wasn't already stiff at the beginning.

Some other examples:

I'm getting tired.

It gets a lot easier if you stick with it.