Living paycheck to paycheck means the funds from the one check are depleted by the time the next paycheck is received. For some, I suppose, they have part of their check go to retirement savings.

But the term generally refers to those where, if the next check doesn't come, life gets difficult. Fast.

These people don't have savings -- they live off their income, and have little-to-nothing to cover an emergency, nonetheless being able to get by for a while if the next check didn't come because of a layoff, or similar.