Teamwork is what happens when a group of people work together well — each doing their part, helping the others, so that the group gets more done than the same people would alone. It is an uncountable noun, so you say 'good teamwork', 'a lot of teamwork', never 'a teamwork' or 'two teamworks'. Careful with the meaning, though, because it is narrower than students expect. Teamwork is not simply 'working in a group'. If five people share a room and each does their own job in silence, that is a group, but nobody would call it teamwork. The word carries approval in it: it points at the co-operating, the passing things smoothly from one person to the next. That is why 'That was great teamwork' is a compliment, and why coaches and bosses say it so often — it praises the way people worked, not the result. You will hear it constantly in sport, in offices, and in school reports, sometimes so often that it goes a bit empty, the way any praise word does when it is overused.