A Key uniquely identifies each individual row in a table.
The key may be a single column (such as Employee ID) or two or more columns (such as FirstName and LastName). Each row in the table must have a key value that uniquely identifies it. In other words, no two rows in a table may have identical key values. This is critical, because it allows AppSheet to reliably find the right table row.
When a user on a mobile device changes data through an AppSheet application, they are making changes to a local copy of the data cached on the device. When the user syncs these changes to the server, AppSheet sends the updated data to the server, finds the updated row using its unique key value, and applies those changes to that row. AppSheet can only find the right row because each row has a unique key value.
Here's more about keys and how to choose one.