Hl1 and hl2 are not really connected plotwise and can be enjoyed separately. There's some small references and a couple characters you wouldn't necessarily know even if you played HL1, because HL1 didn't have much of a focus on that kind of story-telling.

The series is certainly a must play because of how much it influenced the industry. Going back to it now may not seem like it's all that great; but you gotta keep in mind, it was one of the very first FPS games designed this way. Before Half-Life, FPS games were all just basically Doom clones.