The phone notifies you if the phone memory is getting low. To free phone memory you have the following options:
Use File Manager or the respective application in your phone to remove data or to move data to alternative memory as described in the user guide. Typically you first mark files to be removed or moved and then you select the corresponding option (Delete or Move to folder).
Use Nokia Suite to move data to a compatible computer.
To maximize free spaces to your phone memory, you can remove or move the following:
If you are deleting multiple items and any of the following notes are shown: Not enough memory to perform operation. Delete some data first or Memory low. Delete some data, try deleting items one by one (starting from the smallest item). After deleting data, restart (power off/on) your phone.
Tips: Use the memory card or mass memory instead of phone memory in the Camera or other applicable applications in case the selection is available (e.g. via Options > Settings > Memory in use).