For example, δm refers to the change of mass, that is, m1-m2 ...-MX = δ m.
Δ v is the speed change, that is, v1-v2 ...-VX = Δ v.
You will learn a lot in the future,
At least you will use δ when you study the difference method in junior high school chemistry, sometimes it is volume change, sometimes it is molar change, and many, many.