The title of the poem is most likely a reference to the Colossus of Rhodes, a big mamma jamma of a statue that used to stand near the harbor of the ancient city. The Rhodians built the original Colossus to celebrate their victory over Demetrius, who laid siege to Rhodes. It's said this giant statue of Helios, the sun god and patron god of Rhodes, was built in part from the remains of the siege towers and weapons left by the enemy. The gigantic statue gave meaning to the word "colossal," and it was considered one of the Seven Wonders of the Ancient World. Eventually, though, an earthquake rocked Rhodes and the Colossus came a tumblin' down.
In the poem, the speaker describes herself as endlessly tending the remains of a huge fallen statue much like the original Colossus. Many, of course, say this is all one big whopping extended metaphor for the speaker dealing with the loss of her father. It totally makes sense that the speaker would use the fallen Colossus to represent this loss. When it was standing upright, the statue was a symbol of strength, victory, and a seemingly all-powerful sun god. When we're little kids, a lot of us feel this way about our dad. The speaker definitely seems to have almost worshipped him, and seeing him defeated by death seems to have shattered her as much as it did him. By using such an epic allusion to symbolize her fallen father, the speaker makes us really feel the hugeness of loss she wrestles with every day.