You are splitting the sentence up incorrectly. It should be 我的 // 意思 // 是說 // 別緊張 While this sounds weird in English, it sounds very natural in Chinese. You can think of translating it as "what I mean to say is___" Laurence

For example 客人的解释是说这是你的错。The guest's explanation is to state that this is your fault. 我的故事是说我的不整齐的牙齿。My story demonstrates that I have crooked tooth. 我妈妈的话是说男人的应该有一个宽阔的肩膀。my mon's saying is to mention a man should have a broad shoulder. 这封信是说她想念你。This letter is to say that she misses you。 Hope you find it useful.

是"說" = is to say What I meant was not to be nervous 我的意思是别紧张 What I meant was "to say" not to be nervous.我的意思是说别紧张

是說 = that is to say. It functions exactly as in English, for emphasis etc. 我的意思是说别紧张 = That is to say, I meant "not to worry".

是说 is usually used to emphasize what the subject is going to say or express, and is often used in oral speech.