Demand Planner automatically collects data regarding when a product is out of stock. This data helps to estimate the demand correctly. For example, if you sell an average of ten units per month and the product is out of stock for half a month, the real demand is for 20 units. Without taking stockouts into account the forecasted demand could be too low.
By default, your account is set to take into account stockouts. If your store allows overselling (continuing to sell an item when it is out of stock), then you should change this setting to disregard out of stock information.
Enabling/disabling stockout history
You can change this setting for all products at Account > Settings > Forecast.
You can also enable stockout history for selected products in the Replenishment report using Bulk Actions > Set Forecast Settings > Enable use custom forecast settings, then toggle Use stockouts history (note bulk actions only appears after selecting items) or when editing a specific product in it Forecast settings tab:
Manually editing stockout days
The days a product was out of stock can be manually adjusted when viewing/editing a specific variant in Replenishment & Forecast > Edit forecast or in the main Edit forecast screen when the Stockouts Override column is included.