In its basic meaning the word 'beef' refers to meat taken from cows or bulls (wołowina in Polish). But the word also has other meanings which I want to focus on today.
OK, let's illustrate the first meaning of the word 'beef':
On Sunday lunch English people often have roast beef (pieczeń wołowa) and Yorkshire pudding.
Now, 'beef' may also be used as a noun meaning 'power' or 'strength':
His last speech didn't have much beef (moc) in it.
Pull harder! Put some beef (wysil się) into it!
Finally, the word 'beef' also means 'a complaint'. This explains my question in the title...
What's your beef? (O co ci chodzi?)
My main beef (bolączka) about the job is that I have to work at weekends.
And one more thing... the word 'beef' is also used as a verb to mean 'strengthen' or 'make more attractive':
Our team is weak. We need to beef it up a bit (wzmocnić).
The company is experiencing financial problems. However, they're going to beef up their production (uatrakcyjnić).