In Flutter, synchronous and asynchronous calls are two ways to execute code, with important differences in how they affect the app's performance and behavior.

A synchronous call : Is a blocking call, meaning that the app waits for the code to complete before proceeding to the next line of code. In other words, when a synchronous call is made, the app "pauses" until the operation is complete. This can be problematic when the operation takes a long time to complete, as it can freeze the app and make it unresponsive.

An asynchronous call : On the other hand, is a non-blocking call that allows the app to continue executing while the operation is being performed. In other words, when an asynchronous call is made, the app does not wait for the operation to complete before moving on to the next line of code. Instead, the operation is performed in the background, and the app is notified when it is complete.

In Flutter, asynchronous calls are typically used for long-running or blocking operations, such as network requests or file I/O. By making these operations asynchronous, the app can continue to respond to user input and update the UI while the operation is being performed. Asynchronous calls are typically implemented using Futures, which represent a value that may not be available yet, and can be awaited using the `await` keyword.