LESS preprocessor delivers you extend that extend rules in CSS classes. Look at the HTML file:
Let’s see less file:

.mainProp {
    color: orangered;
}
.extProp:extend(.mainProp) {
    background-color:yellowgreen;
}

The less file compiles code into CSS file:

.mainProp, .extProp {
  color: orangered;
}
.extProp {
  background-color: yellowgreen;
}

As you notice, thanks using the extend pseudo-class you can extend rules in one class (extProp) about rules from other class. This other class is given as a parameter of extend pseudo-class. Pseudo-class uses sign ‘:’.

Look at the HTML file:

    
        Animals
        
        
        
    
    
        
My favorite animals
        
Pets
        
Dogs
        
Hamsters
        
And others animals
    

You see as the result:
The effect you obtain if you use instead of :extend after name of selector:   &:extend with parameter inside  the extended class. Look at the changed code less file:

.mainProp {
    color: orangered;
}
.extProp {
    &:extend(.mainProp);
    background-color:yellowgreen;
}

Your CSS file will be the same. The work an extend pseudo-class is similar to the using mixins. Which of them use? If you have to use any parameter, the good choice is  mixins, otherwise the better choice is :extend.