In English grammar there are a lot of terms that are needed to be learned and also understood in order to successfuly identify parts of speech and how they are used correctly. One such thing is the imperritive. An example of the imperrative is 'Please bring me a cup of water.' In the case of the imperrative a command is used.
Labeling a cammand like this one labels the mood of the verb as a command or exhortation. There are usually three types of moods expressed by verbs. The other two are the indicative which makes a factual statement and the subjunctive which indicates doubt.