The MLAT (Modern Languages Aptitude Test) is the admissions test commissioned by Oxford University for anyone applying for a Modern Languages or Modern Languages Joint course (including linguistics). You only have to sit the test if you are starting a language at an advanced level (beginners- don't worry no test for you!). Usually you can take the test in your school (speak to your exams officer) or failing that, another nearby centre.
It will last half an hour per language and in that time you will be asked to translate 5 sentences into the target language, and 5 into English, and you will also be asked to do some quick grammar exercises.
To prepare for it I would recommend starting 3 or 4 weeks beforehand and revise all the tricky grammar points and tenses. With translations, don't worry if some of the vocab stumps you - they do it on purpose to see how you cope when you don't know a word. That being said, do revise some vocab too!