From Bulbapedia, the community-driven Pokémon encyclopedia.

Repel is an item which lets you walk a certain number of steps without encountering Pokémon lower than those in your party. It is manufactured by Silph Co., and is sold in most Poké Marts. The aerosol can that Repel comes in makes it very similar to insect repellant. It comes in three strengths.

Types of Repel

Repel

The most standard and common type of Repel. It keeps lower level Pokémon away for 100 steps and can be bought from Poké Marts for 350.

Super Repel

An improved Repel that does the same job as regular Repel for 200 steps. Super Repel costs 500. It is found at most Pokémarts.

Max Repel

Max Repel is the best Repel (works for 250 steps) and most expensive at 700. It is rare, but is available later in the game.