Note: The following information only applies to plans at a service. We also have help articles dedicated to portfolio planning.
Your planning area is designed for collaboration, so it's possible for two people to have the same plan open and be working on it at the same time.
Two people can work in one plan but you cannot both edit the same cell at the same time. If one person is working in one cell, the other person will see a message on that cell that says that the cell is locked while the other person is editing it.
If a person has tapped on a row and column header in order to add or move it, then the rest of the plan is locked to all others until this process is complete. This is to make sure that one person's changes do not affect the other people who are working on the same plan.