このプロセスのSTATとはどういう意味ですか？

これは実際にはプロセスの状態です。 psで表されるプロセスの状態は次のとおりです。

D Uninterruptible sleep (usually IO)
R Running or runnable (on run queue)
S Interruptible sleep (waiting for an event to complete)
T Stopped, either by a job control signal or because it is being traced.
W paging (not valid since the 2.6.xx kernel)
X dead (should never be seen)
Z Defunct ("zombie") process, terminated but not reaped by its parent.

追加の文字は次のとおりです。

< high-priority (not nice to other users)
N low-priority (nice to other users)
L has pages locked into memory (for real-time and custom IO)
s is a session leader
l is multi-threaded (using CLONE_THREAD, like NPTL pthreads do)
+ is in the foreground process group 

検索を通じてすべてを見つけることができます。マニュアルページの場合、ps特にそのPROCESS STATE CODES部分です。