Learn
“Pay the bills” is a general English expression that refers to a way of meeting expenses. OceanGate, the company behind the Titan submersible, helped scientific, research, and educational companies learn about the deep ocean.
But diving deep into the ocean is expensive and those scientific and educational ventures didn’t pay the bills. That means, OceanGate didn’t make enough money on those trips to meet its expenses. So it turned to adventure tourism, which did pay the bills: adventure tourism could generate enough money to pay (more of) OceanGate’s expenses.
You can use this with a personal budget, too. For example, an actor might spend all night in rehearsals and weekends going to castings. But acting might not pay the bills. His acting work might not be enough to meet his expenses. So, to pay the bills, he works at a hotel part time. (He works at the hotel so he can pay his expenses.)