Coincidentally, an article about what liberalism is today in the Guardian, which is relatively fair.
I very much agree with one thing it argues - that liberalism is so much part of the mental furniture of the western world it is almost the condition of modernity. In the US, "liberal" is now mostly synonymous with "leftwing".
But the most fierce free-marketers are also liberals, from an older strand of classical liberalism. Contrast monarchy, aristocracy, tyranny, communism and the other very different frameworks that most people have lived in for all of recorded history. Compared with them, it is easier to see most Democrats and Republicans as coming from the same broad political tradition.
The Guardian article argues that liberalism is in essence the ideology of the rise of capitalism, and especially the rise of the middle class.
It is very difficult for anyone raised in the English-speaking world to think outside liberalism of one form or another. It seems intuitively obvious to us.
Economics gets most of its disciplinary architecture from that older strand of liberalism - rational choice and optimization, revealed preferences and utility.
The common ground of liberalism is really individual choice - and little interest in the actual aims and preferences people have, the actual choices they make and whether they are able to get what they want. People are assumed to have the foresight and ability to make heir own choices and choose their own purposes.
Liberalism does not really have a good answer for what the common good is, beyond largely procedural "fairness."