This phrase comes from Bethenny Frankel's book, A Place of Yes, and it's the "rule" that stuck with me the most after reading about it. Basically, her point is that regardless of what job you're doing, give it your all because you never know who's watching and what door it could possibly open. Whether it's raising children, losing weight, stuffing envelopes, cleaning toilets, discovering your craft, etc. - make it your business. Take pride in your work, and give it your all. Regardless if it's your dream job or ideal situation.
Maybe your current situation is just a stepping stone, but nothing bad can come from giving it your all. Like I mentioned here, God is constantly networking circles around us, and what might not make sense now will certainly have meaning down the road. Bethenny mentioned that two people that stuck out to her at events she coordinated, one who gave her all prepping food and one who did the same hanging coats, are now living their dreams, one as her personal assistant with open doors out the wazoo and the other as a super successful marketing executive respectively. And I'm pretty sure they didn't think they would be chopping vegetables and taking jackets for the rest of their lives.