This is a potential complement that expresses you "can't do something." Don't confuse it with 不懂 as explained in The "-bu dong" (don't understand) Potential Complement.

Structure
The potential complement 不动 is used to express lack of physical ability to do something. This can be used not only with actions like walking, running or climbing, but even actions like eating and chewing.
It is used like this:
Subject + physical action + 不动

Examples

Wǒ zǒu búdònɡ le，dé xiūxi yíxià！
我 走 不动 了，得 休息 一下！
I walked but haven't gotten there yet. I need to rest a bit.

Děnɡ děnɡ wǒ，wǒ pá búdònɡ le！
等 等 我，我 爬 不动 了！
Wait for me. I haven't climbed there yet.

Zhèɡe niúpái ɡēnběn yǎobúdònɡ。
这个 牛排 根本 咬不动。
I simply can't bite through this steak.

Nà jià ɡānɡqín hěn zhònɡ，tā kěndìnɡ bānbúdònɡ。
那 架 钢琴 很 重，他 肯定 搬不动。
That piano is really heavy. He won't be able to move it.