Mood is a grammatical category that expresses the speaker's attitude toward the action or state of being described in a verb, namely, whether it is a simple statement of fact, a hypothetical or counterfactual situation, a desire, a command, etc. English has indicative, subjunctive, and imperative moods; other languages distinguish more or fewer.
The indicative mood indicates a simple statement of fact: "I'm here."
The subjunctive mood expresses a hypothetical, counterfactual, or desired situation. A counterfactual present situation is expressed in the past subjunctive: If I were king... (which the speaker is not). A counterfactual past situation is expressed in the past perfect subjunctive: If I had been king... (which the speaker was not at the relevant time, if ever). A desired situation can be expressed in the present or future subjunctive: I request that he be there/should be there at noon. God save the Queen! The future subjunctive also expresses a possible future situation in a dependent clause: If I should die before I wake....
In counterfactual statements, the independent clause is expressed in the conditional, which is sometimes considered to be a mood distinct from the subjunctive: If I were king, what riches I would give you.
The imperative mood expresses a command: Go! It typically exists only in the second person and in the first person plural (Let's go!). Some languages, like Spanish, have a separate polite imperative based on the subjunctive.