I agree with the other commenter in general but I’ll give you the bullet points.
Old news:
iMessage is a proprietary chat protocol that only works on Apple devices. Apple has indicated multiple times they have zero intention of porting this to other platforms
Apple users texting each other default to iMessage
iMessage has a lot of useful features over SMS texting that are highly desirable and convenient
On iPhones, when iMessages are being successfully sent in a text, the chat bubbles are blue. If they are SMS, they are green
The US additionally has a weird culture of some iPhone users shaming Android users because of the inability to communicate via iMessage, often referred to by the green/blue bubble appearance
There have been a few attempts to circumvent this, mostly by having a Mac somewhere with software installed to it that forwards iMessages to your Android device, though this is extremely cumbersome as it requires having an entire computer on 24/7 to make sure you receive these messages
New news:
Beeper Mini was released earlier this week, which actually runs a reverse-engineered iMessage client that tricks Apple servers into treating it like an Apple device
It was fully functional for about one day with almost all iMessage features working
Apple made some variety of change on their end that broke Beeper Mini functionality
And that’s about it. For those of us that would like to have easy communication with our iPhone-using friends and family, yet don’t want to change phone ecosystems to do so, this is a problem that would be awesome to see solved.
There are folks that, either because of ignorance or pigheadedness, like to chime in on these threads that they don’t care about having blue bubbles. That is the least important aspect of this to most people following this, for the reasons I mentioned above.