Prioritizing work across multiple projects simultaneously can be very difficult. Having clearly defined objectives and timelines within each project is the best way to figure out how to balance multiple projects. If there are clear objectives, then you can know exactly what needs to be done for each project that you are working on and how important it is to the overall project. A timeline for each project will let you know if tasks need to be done in a specific order or are on the critical path of the project, meaning that there are other tasks that cannot be completed until that is finished. To balance across multiple projects, always start by creating a priority list of your own tasks between any project that you may be working on and map out their tentative timelines. If there is something that is on the critical path of a project, then that should be prioritized compared to a task that may not have anything else depending on it. Having clear objectives for each project to tell you the importance of each task that you are working on should guide you in prioritizing work. 

During my time in industry, I've noticed the necessity of time management in terms of project efficiency and effectiveness. I think this question is fundamental to consider, particularly if you plan on working in a project-based role. For me, it comes down to the priority of each project that I am involved with. The company that hires you has its own business needs, and the priority of each project will certainly trickle down to you as you continue working. It doesn't matter if you complete all of the other lower-priority projects that you are working on if you fall behind on the high-priority one. 

I think a timeline is crucial for prioritizing multiple projects, as mjc22 mentioned. I believe there is a certain level of flexibility in said timeline. While you may hear from a manager that the project must be completed by a certain date, in some cases they may give you just that, and it is up to you to complete all necessary steps before the end date. In other cases, particularly when collaborating cross-functionally, a more rigid timeline for each step may be created for you to follow. I personally prefer creating my own, and I find the use of a Gantt chart can be instrumental in ensuring that all the steps that I need to complete are finished on time.