A purchase charge on a credit card occurs when you purchase things with your credit card and don't pay the balance back in full when you receive the bill.
If you pay off the full balance then you won't be charged a purchase interest charge.
Purchase charges are basically just the interest charged on your credit card when you purchase things and don't pay the full balance off.
You can avoid these interest charges by paying the balance off in full when you receive the credit card bill instead of making payments.