A 'Tie' is a curved line which joins two, or more, notes of the same name and pitch.
When we see two notes joined by a Tie, it simply means that we play the first note, and keep it held down for for the length of both notes.
So, for example, if the note Middle C is a Half note (two counts) and it is 'tied' to another Middle C, which may be a Quarter note (one count), we would play Middle C, and hold it down for Three counts in total ( two counts, plus one count)
It would look like this in written music: