Transliteration is the conversion of one language's script into another language's script.
Let's take this Chinese word as an example: 你好, if we translate this into English, we get: Hello.
If we instead transliterate the same Chinese word into Latin script, we get Nǐ hǎo
So the script has changed, but the language has not. A Chinese speaker reading the Chinese text would say aloud the same thing someone reading the Latin script would say.
Transliteration is sometimes also called Romanization, though Romanization is specifically conversion from a non Latin/Roman alphabet into the Latin/Roman alphabet. Transliteration is more broad and can cover script conversions of all kinds.