Galileo observed phases of Venus that supported heliocentrism. Through his telescope in 1610, he saw Venus change through a complete cycle of crescent, quarter, gibbous, and nearly full phases, much like the Moon.
These observations mattered because Venus's changing appearance depended on its position relative to the Sun and Earth. In the Ptolemaic model, in which Venus always remained between Earth and the Sun, Venus could not display the full set of phases Galileo observed. A heliocentric arrangement, with Venus orbiting the Sun inside Earth's orbit, explained the pattern naturally.
The phases did not by themselves prove every detail of Copernicus's system. They did, however, decisively challenge the traditional Ptolemaic model. The Tychonic system, which kept Earth stationary while having Venus orbit the Sun, could also account for the phases. That is why historians describe the discovery as strong evidence against Ptolemy rather than a complete standalone proof of heliocentrism.