Wipe, in a computing context, means  to render all data on a hard drive unreadable. The term is often used in reference to making data stored on a computer, smartphone or tablet inaccessible before disposing of the device.
A semantic HTML tag is one that defines a meaning to the enclosed content that a computer can be programmed to understand. In most cases, browsers create some visual difference to content in semantic tags, for example, content in the CODE element is usually displayed in a monospaced font. But this is not a requirement. Many people feel that semantic HTML elements are a waste of bandwidth.
In web design, semantics is concerned with the meaning of the tags and attributes that you use to build your web pages. In other words, semantic HTML is HTML that contains meaning in and of itself, beyond any content that it might be marking up. Semantic HTML lets the browser understand what the tag contains, and so will you.