Half Life 2 doesn’t truly need the first one. It adds some context and there’s some callbacks, but you can totally start with 2.
Avoiding spoilers as much as possible, in HL1 something goes wrong at a research facility. Main character fights his way through then gets “knocked out” at the end. HL2 picks up ~20 years later after an entirely new big bad took advantage of the events in HL1 and conquered the world. MC “wakes up” and is dropped right in on a train into an occupied city.
There’s decent plot connections, but you aren’t missing out on anything gameplay wise or largely plot impacting, as the game world has changed so drastically.
All that said, if you want to play HL1 and aren’t interested in it in terms of it as a tech marvel of the time it was created, you can just play Black Mesa. It’s a fan remake that got the greenlight from the original creators to be sold, and by most accounts is a better experience for modern gamers.