The GAE document says that
Because each get() or put() operation invokes a separate remote procedure call (RPC), issuing many such calls inside a loop is an inefficient way to process a collection of entities or keys at once.
I don't understand what exactly an inefficient way means. Can it help me to save write operations?
No it can't reduce write operations. However it can save RPCs.
Think about what is going on. When you call put() the RPC sends data and waits for a response , inside that call is a write to the datastore.
If a single RPC has a 30ms overhead no matter what it is doing (I am making that number up for the sake of the argument) . and the write takes an additional 20ms. Then each put() call takes 30ms rpc round trip + 20ms for the actual write = 50ms.
If you perform 100 put() calls in a loop then it will take 5000ms.
However, if you use put_multi() or db.put([list of entities]) then you will only have a single RPC. This means a single 30ms RPC and 100 * 20ms writes. Which comes out at 2030ms. Ok the numbers aren't accurate and a single RPC with 100 entities will take a little longer than a with just one. But you get the idea.