It's a metaphor. When large, rocky celestial bodies (or "worlds", if you will) crash catastrophically with each other, they are said to "collide". Get it?
It the case of this movie, the proverbial "worlds" are Earth, currently known to science, and Bellus, which is currently make-believe. And, oh yea, not a world, but in fact a "giant star" estimated to be a whopping twelve times the "size" of Earth. I'm sure the astrophysicists in the audience (I have an audience?) appreciate the movie's choice of the scientifically accurate word "size" rather than ambiguous terms like "mass" or "diameter". Awesome job, guys.
The eleventh (and last) movie referenced in the song, Science Fiction/Double Feature. Here's the deal. Scientists discover that a star will obliterate Earth and they try to get people to take the debate seriously. Insert climate change argument here. Here's the good news: Bellus has an earth-like planet (Zyra) that might be habitable. In the span of one year, build a rocket ship to bring all humans to Zyra, and problem solved.
Nothing ever goes as planned. The end result is one rocket carrying a dozen or so principle characters lands on Zyra and Eden is regained. All the stupid humans perish. I hope I didn't blow it for you.
Here are the plot flaws a 21st century geek feels obliged to point out: they had NO IDEA what the surface of this planet was like. Temperature, presence of water, atmosphere, ANYTHING! And when they land, the planet is in the presence of TWO suns, one twelve times the "size" of Earth and the other, our sun, over 330,000 times the mass of Earth. And the sun of choice just had a planet crash into it. I'm guessing there was going to be some serious solar activity that day!
That said, I liked this movie. It moved a little slow and the acting was rather wooden, but quirky enough. I rank it behind "Forbidden Planet" and way behind "The Day The Earth Stood Still", but I'd watch it again. At least for the matte paintings. I suppose.