First, some surrounding context:
When Zarathustra was thirty years old, he left his home and the lake of his home, and went into the mountains. There he enjoyed his spirit and solitude, and for ten years did not weary of it. But at last his heart changed,--and rising one morning with the rosy dawn, he went before the sun, and spake thus unto it:
Thou great star! What would be thy happiness if thou hadst not those for whom thou shinest!
For ten years hast thou climbed hither unto my cave: thou wouldst have wearied of thy light and of the journey, had it not been for me, mine eagle, and my serpent.
But we awaited thee every morning, took from thee thine overflow and blessed thee for it.
Lo! I am weary of my wisdom, like the bee that hath gathered too much honey; I need hands outstretched to take it.
As @eques notes in their answer, the fragment in question can be rephrased as "took your overflow from you", and while it loses a certain poetic tone if you do so, the primary meaning is the same.
Zarathustra is addressing the sun. The "overflow", then, is the light and heat it provides - each day, it shone into his cave, and he (and his eagle and his serpent) accepted it ("took from thee [it]") gratefully ("and blessed thee for it"). This is the literal level of the text, but he proceeds to draw a comparison to his own accumulated wisdom.
Zarathustra says - presumably metaphorically - that if the sun had not unloaded some light into his cave, it would have grown tired before completing its journey across the sky, and that in the same way, he must go and distribute his accumulated wisdom.
All of that being said, if you're learning English as a second language, you might want to read a translation into your own first language. Thus Spake Zarathustra wasn't originally written in English, after all, but in German, so a direct translation (or perhaps the original, if you already speak German) will give you a better sense of the meaning, if one is available. Every translation necessarily changes at least some small part of the meaning, so trying to translate a text in your head which is already a translation will give you less sense of what Nietzsche intended than a translation directly from the German will.