File System Triming

File system triming can help optimize write speed by cleaning data marked for deletion, that would otherwise cause lags while writing further data.

NB: On most recent Android versions the OS deletes data immediately and automatically, in which case you'll see a "not needed" after trimming partitions.

In this case you can hide this tab by long-pressing the tab title.

On Android, actual deletion is skipped to reduce battery consumption, however this can cause huge lag while writing further data: flash memory is written by entire cluster and if any data already exists in a cluster it will have to be read before being actually written. So if a cluster is not empty because of data marked for deletion this will slow down the write process.

FS triming ensures data marked for deletion is actually deleted.

Here you can find more information about read/write methods on flash/SSD disks. Note however triming should not and is not activated on flash memory on Android.