Transliteration is a conversion of a text from one script to another. It swaps letters (thus trans- + liter-) in predictable ways (such as α → a, д → d, χ → ch, ն → n or æ → ae.
Transliteration is not about the sounds of the original. It is about the type or written characters, mostly the letters.
Example: the name for Russia in Cyrillic script, "Россия", is usually transliterated as "Rossiya". So, 'сс' is transliterated as 'ss', but pronounced /s/.
Transliteration typically goes grapheme to grapheme. Most transliteration systems are one-to-one, so a reader who knows the system can reconstruct the original spelling.
Transliteration is opposed to transcription, which maps the sounds of one language into a writing system.