In short
Capacity planning is the process of checking whether a production plan is achievable with available machine, line and labour capacity, and balancing the load.
Even when materials are ready, a plan fails if capacity is short. Capacity planning shows the load on work centres over time and exposes bottlenecks.
Infinite capacity planning only reports overloads; finite capacity scheduling places jobs in a realistic sequence within capacity limits. Including maintenance plans in capacity makes the plan match the shop floor more closely.