A translation management system (TMS) is software that holds every part of a translation project in one place: the source files, the translations, the memory of everything you have already translated, the approved terminology, the quality checks and the delivery back to the client.
Instead of a folder of documents, a spreadsheet of terms and an inbox full of file versions, a TMS keeps the text as segments. Each segment is checked against your translation memory, pre-translated where a match exists, flagged by QA as you type, and exported back into the original file format with the formatting intact.
The category grew up serving large localization departments, which is why most systems are sold per seat and per year. WordBeam is the same category of tool, built around a single translator and their clients.