Let's do an experiment. I'd like you to answer this question: what is a project? I'm not asking in the sense of "Do you know the official and correct definition of the word?" I'm asking what you think of when you use the word project. Take a moment to think about it and write it down. Now go ask one of your colleagues to do the same thing. Put the two pieces of paper side-by-side and see what you find.
You will almost certainly see that you will have two different definitions of the word. Talk about them for a moment and you'll probably find that you can agree that you are talking about the same thing. One of you may have something like "A series of tasks the produce a change." The other might have something like "An objective that has to be achieved." Different, but not that different, right? Not exactly. You'll find the definitions range from "A task," all the way to "An Objective", and while one usually implies the other, these are two very different things.
You can repeat this experiment with random people you meet on the street (I'd recommend you don't because people will tend to give you odd looks and shuffle uneasily around you without speaking) and if you ask 10 different people, you'll get 10 different definitions of the word. Again, with some discussion you could probably agree you're talking generally about the same thing, but you can't ignore the fact that the definitions are different.
Now ask yourself another question: what might happen if a group of people is assigned to work together on something called a 'project' and they all come to the table with a slightly different understanding of just what a project is in the first place? One person is thinking s/he needs to define some tasks they need to do in the next week, while another is entirely focused on trying to understand the objective they need to accomplish and sees no value in talking about tasks yet.
To make matters worse, what if these people don't start off by first agreeing on a what a project is before moving on to tackle their specific assignment? In a word: conflict. Not necessarily out-and-out, in-your-face, fisticuffs conflict, but a subtle, slow-burn-frustration kind of conflict. The kind that leads to irritation and dis-engagement. And frustratingly for the people on the team, they won't really understand why the conflict exists. They often think they have an issue with their colleagues' interpretation of the current assignment when what they're really struggling with is the underlying process by which they will work together to accomplish the assignment.
When we don't share a common language, a common process, and common tools, the best intentions in the world are not enough to create success. In my experience, we too often make the assumption that we're working from a shared understanding of how we'll tackle a 'project', without ensuring that we first understand the process by which we'll work together. When things start to go wrong, there is no way to address the issues if we don't have an understanding of the causes.
Next time you're invited to join a project team, start by agreeing on the overall process before you tackle anything to do with the specific assignment.