On social media services, the symbol "#" is called a "hash tag". People use hash tags to mark the topic of a social media message.
The phrase "hash tag" also refers to the combination of that symbol with a word. For example, if you're writing a message on Twitter about learning English, you could add the hash tag "#English" to the end of your message.
Outside of social media, the "#" symbol is called the "pound symbol". You might hear this if you call a company with an automated customer service system:
Press "pound" to hear more options.