Why is there a "P" on my photo/avatar in Microsoft Teams?

This just means that your account was selected for "Public Preview." The Microsoft Teams Public Preview channel allows customers to easily preview and evaluate pre-release Teams features before they roll out to the rest of their company. Essentially, you are trying features not fully released to all customers.

See this link for more info:

https://techcommunity.microsoft.com/t5/microsoft-teams-public-preview/microsoft-teams-public-preview-indicator-p-on-avatar/m-p/2734037