般若波羅蜜多心經 Mantra : 觀自在菩薩 (观自在菩萨) Observing the Avalokita, the Holy Lord and Bodhisattva was in deep and relaxed Meditation

觀自在菩薩

"觀"自在菩薩, The first character of "觀" is generally understood as meaning Observing but "觀" could it also mean "觀世音菩薩" (Guan Yin). As because the subsequent words in the 心经 "Heart Sutra" describes how Guan Yin has spoken to Gautama Budha's deciple 舍利子. 

So, I could interpret 觀自在菩薩 as when Gautama Buddha was about to attain Nirwana (Enlightenment) where his disciples (including 舍利子 who is Gautama Buddha's first disciple) are closely at his side observing their peaceful Master in deep meditation. And it is at that same time that Guan Yin (the previous Buddha) arrived at the same location to summon words of wisdom to Gautama Buddha's followers.

It was a significant moments when Gautama Buddha has attained Enlightenment, and Guanyin; the previous Buddha descended upon Gautama's disciples to, or perhaps to the worldly beings to remind of the true meaning of the ending of the life-death-rebirth cycle.

(the above are summarized and found via Web research and Article Readings, it is only for my reference in the Study of the Heart Sutra)

心經 The Mantra : 觀自在菩薩

《心经》

觀自在菩薩

「觀」，是定下了心,  是指“心境平和"，“态度冷静” 思想的时候。(「觀」Guān, means a settled heart and when one is in the state of mind which is peace and calm)

「自在」，是"無可"和"無不可"的自在，是一种心态，"有也可以" 即使 "没有也可以" 的一种心态, 应该不是讲 "自由", 而是讲来自内心的 "自由自在 "。因为这种 "自由" 是不肯受拘束的，但往往我们的環境是不如我们的意思，便苦了自己。獨有"自在"，是无处不安，自然地不受環境所困，一切好壞隨緣。(「自在」Zìzài, is a state of mind of which "with" or "without" is accepted in the hearts and mind, it is "free at last from within" and no more worldly bindings. The surroundings will no longer affect us anymore. )

「菩薩」，是"菩提薩埵" (梵文 "Bodhisattva") 的簡稱 Bodhi（菩提）意為「覺悟」, Satto 或 Sattva (薩埵) 意為「有情」; 漢語的意思為 "覺悟的有情眾生" , 能自己覺悟，再能覺悟它人的謂之"菩薩"。(「菩薩」Púsà, is from Sanskrit "Bodhisattva" in short "Bodhi" or 「in the State of Self-Realization」,  when the one who had attained self realization, then only one can assist others in attaining realization is called 「菩薩」.