Turbulent flow refers to a very thin turbulent layer that hugs the airfoil skin. This "rough" layer helps the smooth laminar flow above it stick to the airfoils shape better.

A true 100% laminar flow that hugs the airfoil shape for it's whole chord is the ultimate but it's very hard to maintain under the entire operating range of the airfoil. At high speeds it's not that hard to accomplish but as the speed drops, the lift coefficient goes up and the air has to "work harder" to follow the shape it will tend to separate in a big bubble on the upper mid to rear areas. If it re-attaches before it comes off the trailing edge then some of the damage is minimized but the model will typically be considered as "mushy" while the bubble is there. Dropping the nose a bit and speeding up will make the bubble collapse and return the feel to "slippery and responsive". If the bubble does not re-attach then it's considered as a stall. Laminar flows typically transitiion from smooth to bubbled or stalled very quickly and this gives the model a reputation for being a snap roller or nasty staller.

Using an airfoil that promotes a thin turbulent layer either with shape, construction design or with turbulator strips will force a thin turbulent layer to form before the separation bubble can form. If done right this turbulent layer will greatly delay the separation bubble formation and delay and soften the stall charactaristics. Yes it is a bit more drag than a true laminar flow but most folks consider it a good tradeoff.