A translation memory, or TM, is a database that stores so-called "segments", which can be sentences or sentence-like units (headings, titles or elements in a list) that have previously been translated. A translation memory system stores the words, phrases and paragraphs that have already been translated, in order to aid human translators. The translation memory stores the source text and its corresponding translation in language pairs called “translation units,” often abbreviated as TU.
For more details about translation memories, see the Wikipedia article.