Peeking consists of checking the results of an ongoing A/B test and stopping it as soon as the p-value falls below the threshold. This practice multiplies the opportunities to get a false positive and pushes the real type I error risk well beyond alpha. The remedy is to fix the sample size in advance or to use sequential methods.
Example
Looking at the p-value every day and stopping on the first day when p < 0.05 produces false winners.