save

Zoos save animals.

To 'save' an animal is to protect it from danger or death.  The example says that zoos keep the animals safe from death.  Notice the following:

1. He saved my life last year.
   
2. We need to save more water.
   

population

Zoos help to make animal populations grow.

The animal 'population' is the number of animals there are.  We can also use this to talk about the number of humans there are in a city or town.  Notice the following:

1. What is the population of your city?
   
2. The population is increasing every year.
   

definitely

Zoos can do some good things, definitely.

'Definitely' is similar to certainly or for sure.  We use this for something that is a fact.  Notice the following:

1. He will definitely come to the party.
   
2. We can definitely take care of your daughter on Friday night.
   

pet

I love cats, just as a pet.

A 'pet' is an animal that lives in your house with you.  You care for it and give it food and love.  Notice the following:

1. He keeps a snake as a pet.
   
2. Do you have any pets?
   

in the wild

Have you ever seen a monkey in the wild?

If an animal is 'in the wild' it is in its natural habitat or home, not in a cage at the zoo.  Notice the following:

1. Didn't you see a shark in the wild last year?
   
2. At the end of our hike we saw a bear in the wild.