Titration is an experiment you use to find the concentration of a substance (e.g. Hydrochloric acid). I think that it doesnt come up in unit 2 but it definatley comes up in unit 3. If you wanted I could make a resource showing how it works. However I don't think you need it because the revision book I have which goes exactly through the syllabus for AQA has got titration in unit 3
By the way the revision guides that I use for science are CGP books that are made especially for the AQA exams. They are really useful and tell you exactly what you need to know for the exams. They really help!