Tied notes are two notes of the same pitch joined together with a tie. A tie is a curved line that links the two notes together. The two tied notes are played together as one single note with a longer duration. For example, a half note and a quarter note of the same pitch tied together has the same duration as a dotted half note. A dotted half note is equal to the sum of the two tied notes (a half note + a quarter note).
Piano Music Theory 