In a piece of music that I played, in one measure there was a slanted line connecting two notes, kind of like a slur, except it was straight and it was diagonal, and it wasn't touching the notes, just connecting them.
The straight line could be a glissando or a portamento.
A portamento is a smooth change in pitch - like you'd do on a trombone, violin or swanee whistle.
A glissando is what you'd do on an instrument like a piano or xylophone (instruments that only allow discrete pitches to be played) to give the impression of a portamento. You can't do a smooth pitch change on a piano, but you can do a smear by running your thumb up or down the keyboard to some destination note, or start on a note a smear away from it.
As you're playing this on viola, I'd suggest performing this as a portamento (you can't really glissando on a non-fretted string instrument).
While I'm here, the supplied viola part is not notated in a helpful way. It starts like this, and because the rhythm hasn't been beamed to group beats together, it's impossible at a glance to see whereabouts the quarter note in the first bar should start:
Although it sounds identical, it's far easier to read if the notes are beamed like this. Music should be typeset so it's as unambiguous as possible. It shouldn't make you go "huh?"
That does not look as much as a slanted line between notes than it does like a slanted line before notes. Basically you slide from an unspecified pitch below (likely open string or a bit above it) into the respective given pitch, on three successive strings it would appear. Which strings to choose may depend on the position you want to end up playing.