In one sentence, describe what the following article is about:

It's the light-blue app with a white bird. Log in with your email, phone number, or username and password, if you aren't logged in automatically. It's located in the top-right corner of the tweet. Promoted tweets have a "Promoted" tag at the bottom. Instead of "account",  this will display the Twitter account of the company or service that promoted the tweet. This will open a pop-up window asking you to confirm. Blocked accounts will not be able to follow or message you and their tweets and ads will not appear in your timeline.  Tap Undo in the bar at the bottom to immediate unblock the Twitter account. You can always unblock a Twitter account later at any time.

Summary:
Open Twitter. Tap  on the tweet. Tap Block @account. Tap Block to confirm.