underscored note?

[henrikhank]

What happens to a note if has a line under it (it is underscored) and what is it called?
An example: https://www.piano-play-it.com/images/sixteenth-note-piece.png

[lloyd_cdb]

It's a tenuto.  It's the same as the one's earlier in the piece that have it above the note, it's just below because of the direction of the staff.  I usually take is as a slight dynamic emphasis, without it defining a melodic line like when you use an accent (>).

[quantum]

Yes, tenuto. 

You should know Bach didn't litter his scores with such verbose articulation and dynamic markings.