Cello rosin is what allows your bow and cello to make sound. Like a bike chain won’t move without grease, a cello bow won’t make sounds against the strings without rosin. The stickiness in the rosin allows the bow to “catch” on the string, pulling it ever so slightly to cause vibrations.
There are two main types of rosin: amber and dark. Dark rosin provides a softer tone and is better suited to cool, dry climates since in warmer climates it gets too sticky. Dark rosin is generally used by cellists who need a softer rosin to make the cello sing. Lighter, amber rosins tend to be harder and more dense making them a good fit for cello.
Some rosins even have precious metals inside. Gold rosin is said to produce a warm, clear tone and is appropriate for all instruments. Silver rosin creates a concentrated, bright tone and is especially good for performance in higher positions. It is best suited for the violin or cello.
The next major decision you need to make when choosing rosin is boxed or caked. Most students use boxed rosin to begin with as it is often included with many beginner cello outfits. It is lower quality and can be used by any instrument during any season. In addition, it is less prone to cracking. Caked rosin is used by more advanced players who want a better sound.