The Speech Synthesis Markup Language (SSML) is a standardized markup language that provides a rich, XML-based language for assisting the generation of synthetic speech. It's used to improve the quality of text-to-speech synthesis by providing more control over pronunciation, volume, pitch, and speed of the synthesized speech. SSML is significant, as it enhances the user experience by making machine-generated speech sound more natural and understandable.

Here are some SSML tags:

• <speak>: The root element of SSML.
• <p>: Describes a pause in the speech.
• <s>: Represents a sentence.
• <break>: Specifies a pause or silence.
• <prosody>: Controls aspects of speech such as pitch, speaking rate, and volume.
• <emphasis>: Indicates the strength of emphasis to be applied to the enclosed text.
• <say-as>: Allows for indicating the content type of the text to improve pronunciation.
• <phoneme>: Specifies the phonetic pronunciation for the contained text.
• <sub>: Allows for specifying a substitution to be performed by the processor.

Now, let's look at an example of an SSML document:

<speak>
    <p>
        <s>Hello, <break time="200ms"/> my name is <say-as interpret-as="characters">AI</say-as>. </s>
        <s>I can <emphasis>change</emphasis> the way I talk using <prosody rate="slow" pitch="-2st">Speech Synthesis Markup Language</prosody>.</s>
    </p>
</speak>

In this example, the text "Hello, my name is AI" is split into two parts with a 200ms break. The word "AI" is spelled out character by character, the word "change" is emphasized, and the phrase "Speech Synthesis Markup Language" is spoken slowly and at a lower pitch.