What is A/B Test?
An A/B test is an experiment that splits users randomly into two groups, shows each a different version, and compares a chosen outcome.
A/B Test explained
Random assignment is what lets you attribute a difference to the change rather than to who happened to see it. You decide the outcome measure (for example sign-up rate) and how long to run before looking.
Stopping the test the moment the numbers look good inflates false positives. Decide the sample size and the end date in advance, then test whether the difference is statistically significant.
Example
Half of visitors see the current checkout page and half see a shorter one; the test compares the share who complete a purchase.