This means:
“Learn a method that supposedly removes a very large amount of waste—claimed to be 15 kilograms—from your intestines/colon overnight.”
Breakdown
“Learn how to” = the headline promises to teach you a particular method.
“Clean your colon” = supposedly remove stool or other material from the large intestine.
“15 kilograms of waste” = claims there could be an enormous amount of waste inside the colon that can be removed.
“In one night” = suggests this can happen extremely quickly.
Tone
This is a sensational health/detox headline. The “15 kilograms” and “one night” claims are designed to shock people and encourage them to click.
The important point is that the body does not normally store 15 kg of “waste” in the colon, and you don’t need routine colon cleansing or detoxes to remove supposed accumulated toxins. Some colon-cleansing methods can actually cause dehydration, electrolyte problems, or other complications.
A more accurate version would be:
“What you should know about colon cleansing and overnight detox claims.”
