Can somebody briefly explain Black Mirror? Are they each individual horror / sci-fi stories?
Like others have pointed out, it's an anthology series. Each episode is its own thing. Some episodes run like 40 minutes are so. Some are 1:20+, basically like a feature film. In any show like this, it is inevitable that some episodes are going to be better than others, but (a) the quality of this show is superb and pretty consistent across the first handful of seasons and (b) if you don't like a particular episode, no biggie.
If you decide to start this and are unsure about whether it is for you, I very strongly recommend NOT starting with S1E1 (The National Anthem). That is a very good, highly relevant episode. But the subject matter is extremely off-putting to viewers who don't know what they're getting into. You can come back to this one once you're a few episodes in. The next two episodes -- 15 Million Merits and The Entire History of You -- are both 10/10 episodes that accurately signal how this show operates. If you watch those and don't like them, you can comfortably skip Black Mirror. But you'll probably like them.
Edit: Actually, The Entire History of You is arguably the single best episode in the entire series. Everybody has their favorite, and if we did a poll I'm sure San Junipero and maybe a couple of others would give it a run for its money, but that episode is firmly ensconced in pretty much everybody's Top 5. And it requires no set-up and follow-up. If you don't like it, Black Mirror probably just isn't for you, which is fine obviously.