Skype is a service that uses the internet to make calls and send instant messages to other people using Skype all around the world.
Don't forget, downloading and using the Skype app uses data, which will come out of your data allowance.
If you have a Pay As You Go phone, you'll need to top-up and buy an Add-on that has internet allowance so you can use the internet to download Skype. Using the Skype app will use data but you can connect to Wi-Fi to avoid being charged. There may also be some other charges.
Get more support for Skype here.