Tuning is important for every instrument because most music will not sound good if it is not in tune. String instruments like the cello use pegs for tuning. The string is wrapped around the peg, and you turn it one way to loosen or lower the string and another way to tighten or make the sound higher. Pegs can often slip, which causes the pitch to go down and problems with intonation, but there are ways to prevent this.
Tune your cello. When you tighten a string, push the peg slightly into the head, which will help the peg stay in place.
Add a light coat of a peg compound if your pegs still are slipping. Do not use too much of the compound because it can build up.
Rub some candle wax on pegs that get loose. This fix is similar to the peg compound, and you should also use it sparingly.
Take your cello to a luthier or repair shop if your pegs continue to slip. The luthier can adjust the size of the holes where the pegs fit in the head or switch to a peg with a thicker shaft to help keep it from slipping.