Best clarified by Wikipedia
"A Turtle strategy is commonly used in real-time strategy video games. When turtling, the player protects his territory, to the exclusion of creating any forces for attacking the enemy. The Turtle tactic is a common counter to the Zergling rush tactic.
The most common way to turtle is to build massive amounts of towers to fire upon enemy units, however these mass groupings of towers are easily countered by longer ranged or heavily armored siege units that deal large amounts of damage to buildings...."