I’ve not heard of the term “pork barrel” money before, what is meant by that?
Let's say you and I are both congresscritters. You want to expand spending for social security, but you need other congresscritters to vote for your bill. You come to me, but I don't particularly care about social security, but I might really care about education. I tell you I might vote for your bill, but only if you put in something extra into the fine print about education.
The bill is obstensively still about social security, but if passed also now does something about education, which makes me happy.
Its a form of compromise but often obfuscates what bills are really about to the public, and who/why people support certain bills. To the public I might be against social security, but I voted yes on this social security bill, because hidden away was really what I wanted.
I would like to emphasize that as Pork Barrel is traditionally used, it refers to... well again, let us take an example as, oh say, Joe Manchin of West Virginia. Joe Manchin is a Democratic Senator representing a Deep Red state, where Trump won every single county in 2016 and had his largest voting share of any state, at +41.7% in 2016 (down to a mere +38.9% in 2020). There is a debate over a bill in the Senate on the issue of, let us say Social Security as in the example above. Manchin says nay, because Socialism and it puts his voters into a frothing rage.
And so to get Manchin on the side of the bill, it is amended to also include funding for projects in West Virginia; perhaps a bridge, perhaps repairing a highway or other public works project; a special Defense contract of some kind like for building planes perhaps, or agricultural subsidies. This pleases Manchin, and when he goes home he will bring news of the many benefits he was able to bring to West Virginia that he could only get by being a Democratic Senator and not a Republican, and this gives him cover to vote for something West Virginians might otherwise vote him out over. Other popular examples are convincing members of certain house or senate committees (without which a bill cannot advance to the floor) through pork barrel; this is somewhat more common as individual votes are more frequently relevant there.
The issue is, of course, that it means taxpayers fund something for the benefit of one state (or district) and not on the basis of whether they really need it, but instead on the basis of political expediency. It is an example of the maxim that "All politics are local" in that whatever a senator or representative may believe, they are ultimately elected by a specific locale and thus beholden to the interests of that place over any ideological ones.