At some time you might want to become a member of some on-line service, (e.g. a messaging service for a social club, buying and selling on eBay, paying your bills on-line, etc.). The process usually involves setting up an account with them, and providing them with some method to contact you, to verify that you're a real person. Commonly, this might involve the following steps.
Visiting their website and following a “join up” link.
Filling in some details about yourself (name, age, location, your interests etc.).
Creating a username and password that you will use with their service (use different ones than you use elsewhere, or use a different password, at the very least). Do not give outsiders the password to your account on your internet service provider, nor the password to your email account.
Giving them a contact address to verify that you're a real person (e.g. your email address, or phone number).
Waiting for them to contact you, then following the directions they give you to confirm your membership (which probably involves clicking on a link in an email, and/or entering a code or password that you've been told to use).
A trial period where you have limited access to their service, until they're satisfied that you're bona fide.
Thanks to wide-spread abuse of internet services, most have to take steps (like the above) to ensure that members joining them are a real person, and not a machine. That it's someone who's going to make use of the service for the purpose that it was set up for, and not to just make a nuisance of themselves. And to be able to block anyone, or anything, that doesn't fit in.
The converse applies—there are some services that are set up for no purpose other than to scam people. For reasons like that, you have to take some precautions before joining up to a service. Try and find out if they're for real—do an internet search against their name or website address, adding a keyword like “scam” to your query. Do not rely on personal recommendations, you'll find that most people who recommend something know very little about what they're recommending. Even if you don't find anything bad about them, be cautious about what personal information you provide. And never give a third party details that they could use against you, such as giving them your logon credentials that you use for your private mail. There's a number of scams that ask you for those details, then they logon to your service as if they were you, and send spams out in your name, perhaps to everyone on your contact list.
To guard yourself against being abused, you should always use different passwords on different services, so that if one gets compromised, the rest are still safe. And using different usernames on different services, makes it much harder for scammers to guess who you are, elsewhere.