Skip counting by 5s (5, 10, 15, 20...) establishes a pattern useful for telling time, counting money (nickels, dimes), and multiplication. The pattern ends in 5 or 0.
You already know how to count one by one: 1, 2, 3, 4, 5... Skip counting is a faster way to count when things come in equal groups. When you skip count by 5s, you jump over four numbers each time and land on every fifth one: 5, 10, 15, 20, 25, 30...
Notice the pattern in the ending digits: 5, 0, 5, 0, 5, 0. The numbers always end in either 5 or 0, never anything else. This is a helpful check — if you land on a number like 23, you made a mistake somewhere, because 23 does not end in 5 or 0.
Skip counting by 5s shows up in real life more than you might think. Look at a clock: the minute marks go 5, 10, 15, 20 all the way around. When you count nickels, each one is worth 5 cents, so five nickels = 5, 10, 15, 20, 25 cents. Any time you have groups of 5 things, you can skip count instead of counting every single one.
Practicing this pattern also builds your foundation for multiplication. Later, when you learn that 5 × 4 = 20, you will already know that because you have said "5, 10, 15, 20" many times. Skip counting is multiplication in disguise.