Tied notes are two notes that has a curved line tying them together. What does it explain to you? When to hold a communication over a bar line. In 4/4 time, a bar consists of four notes. What happens when you need to hold some text over on the last beat with the first bar on the first beat on the second bar? You can’t write a half note in the middle from the bar, what exactly you do is write two quarter notes and tie them together.
You will likely see harder tied notes, say for example a quarter note bound to an eighth note. The same logic applies there. The note could well be held for your length of 25 % note plus the amount of an eighth note. You will see slideshow fair amount in melody lines when you start playing harder songs, so be sure you understand tied notes.