Phone verification
For example, you may get an email like this:
Stripe may also show what verification steps you need to take when you are logged into their dashboard, for example:

ID verification
You may also be asked to provide ID verification.
For identify, the list of documents accepted by Stripe are listed here:
You can view what has been requested and upload documents here:
https://dashboard.stripe.com/account/documents

Company ownership verification
You may also be asked to verify company ownership. A list of documents that can be used to verify ownership, by country, are listed here:
You can upload ownership verification documents here:
https://dashboard.stripe.com/account/verifications

More info on verification
Read more about Stripe identity/company verification here.