Disclaimed: My speciality is more in the Early Modern English side

Originally, in most or all countries in Western Europe, aristocracy began as an extension of some forms of chieftains, warlords and such. Local leader, particularly who could call upon military force of some sort. Aristocracy makes this long-running (not a potentially-democratic role) and inheritable and gives it perhaps a little more pomp. As aristocracy developed many lords were originally retained or other higher lords or monarchs (themselves essentially higher lords). As in England, where many of the older aristocracy and gentry descended from Norman retainers of William and his immediate descendants, particularly; grand families usually claimed descent from knights. Aubrey de Vere I was a chamberlain and local justiciar who's grandson Aubrey III (not terribly original with names, the English aristocracy) became Earl of Oxford and as well as ranking highly as earl they are one of the oldest earldoms in England making them very high ranking.

Extending from such origins, the original role of lords was very much as active landowners, local leaders and dispensers of justice and military leaders who would raise their followers as part of the nation's armies or militias. The distribution of respective titles in European hierarchies is based on the size and position of their land, which is one reason the titles are relatively similar, u/mimicofmodes covers this hierarchy here with reference to England https://www.reddit.com/r/AskHistorians/comments/i6jxos/what_were_tudor_era_titles_actually_worth/

As mimic notes, overtime the degree of involvement with local governance was diminished. Nobles I would note in the Early Modern did still generally have lands, expect to hold local offices, and would frequently be major army officers. But they became less local as the centre of political gravity strengthened on court and parliament and they sought involvement in the business there (including the increasing importance and sophistication of national ministerial government).

That's the general idea. Of course the origin, exact timeline and particular context varies across different countries, and different individuals were always varied in exact roles and behaviour.