This is the most common advice given to new writers. The is terrible advice. For example, two excellent thrillers I recently read, (7th Heaven and The Secret Servant) happily disregard this sage advice. The proper advice is Show or Tell. As the plot unfolds, the author must decide to either show or tell. If the two are combined, the readers will be bored by the redundant exposition. When an action in abridged with a telling summary, the emotional commitment is lost. When a telling summary is extended with a showing digression, the pace is lost. The author must commit to either Show or Tell.
Here is an example from classic Elmer Gantry by a Nobel Prize winning author. After dedicating two chapters to show us Reverand Gantry's first church. The next short chapter begins by telling:
A year he spent in Rudd Center, three years in Vulcan, and two years in Sparta.Six years of plot gone in one sentence! This is the power of telling!
For each step of your plot, make a decision: Show or Tell, both not both.