What does the blue tick mark beside people's names on twitter?

[i love you]

And usually I find then beside celebrity accounts
Do they like have to pay or something to get that done or can't anyone just get that done or pay to get it done like how does it work or do you have to actually be famous and known to some how for them to let you get it done

[Cassia]

It means they have a verified account. Many celebrities and other public figures have their accounts verified, which means that they really are the person they say they are. Not just some random person who wanted to get a bunch of followers by using a celebrity's name.

I don't know if they have to pay for it, or how it's done, but I hope I helped you a little bit.

[Leslie]

Like the person above said, it means the account is the real person, and not some fake account. I don't know all the ways to be verified, but I know one is having an official website.