Mandarin is used in both Taiwan and Mainland China, and people from both places can usually understand each other. But some characters and daily words are different.
In Taiwan, people use Traditional Chinese characters. In Mainland China, people use Simplified Chinese characters. Sometimes the pronunciation is the same, but the characters look different. Tap the speaker icon next to each example to hear it.
These differences are normal — a little like the differences between American English and British English. The words may be different, but people can still understand each other most of the time. They are simply different ways people use Chinese in daily life.
If you are planning to spend time in Taiwan or Hong Kong, Traditional Chinese will be more useful. If you are going to Mainland China, Simplified Chinese will be more useful. For speaking, it is best to learn the everyday words used in the place where you will spend the most time.