Tied notes are two notes with a curved line tying them together. What does it tell you? When to hold a note over a bar line. In 4/4 time, a bar is made up of four notes. What happens when you want to hold a note over from the last beat of the first bar to the first beat of the second bar? You can't write a half note in the middle of the bar, so what you do is write two quarter notes and tie them together.
You will also see more complicated tied notes, such as a quarter note tied to an eighth note. The same logic applies there. The note would be held for the length of a quarter note plus the length of an eighth note. You will see this a fair amount in melody lines once you start playing more complicated songs, so make sure you understand tied notes.
Next you can learn about piano rests .