The phrase "to thicken the smoothie" means making the smoothie thicker or more dense in consistency by adding ingredients or using certain techniques. Full definition
It helped thicken the smoothie nicely too as sometimes mine get a little watery. (rockmyvegansocks.com)
The fruit will add flavor and nutrients, as well as help to thicken the smoothie without the addition of sugary frozen yogurt or ice cream. (healthyeating.sfgate.com)
If you are used to making high carb, fruit filled smoothies, you are probably thicken your smoothies with banana or maybe even mango. (essentialketo.com)