what is i-551 stamp mean

please

The I-551 stamp is normally received at your AOS interview.
The I-551 passport stamp is needed if you wish to travel outside of the USA when you are a permanent resident, but you are waiting for your green card in the mail.It also gives you work authorization, if you didn’t apply for an EAD (Employment Authorization Document).

It means you are Green card holder... Actual Green card is in the mail.