The boost button is available for every listing in order to push it to the top of the main feed for optimal visibility. Every time you boost, a timer begins counting down to the next time you are eligible to boost your listing again.
You can find the boost button by going to your Profile > Listings. You'll see the button alongside your list of items or you can tap an individual item and the boost button will appear in the bottom left-hand corner.
Each listing can be boosted, for free, 3 times. The number free boosts available appears within the boost button and will automatically adjust as each boost is used. Once you use up all the free boosts, you can purchase a boost package if you'd like to boost your item again.