Abstract nouns designate things that can only exist in our imagination; they cannot be touched or seen (like the concrete nouns).
Examples of abstract nouns are:

feelings
la felicidad - happiness
el amor - love
la duda - doubt

virtues
la inteligencia - intelligence
el valor - valor
la gratitud - gratitud

sciences' names
la química - Chemistry
la física - Physics

faults
la flojedad - slackness
empeoramiento - deterioration
debilidad - weakness

Example sentences:
A Licha le dio mucha tristeza que yo hubiera vendido el piano.
Lisa was full of sadness because I had sold the piano.

Tenemos suficiente juventud, necesitamos una fuente de inteligencia.
We have enough youthfulness, we need a fountain of intelligence.

¿Has escogido estudiar la carrera de música o la de ciencias políticas?
Have you decided on studying Music or Politics?

Pedro confesó su amor por Lucía.
Pedro confessed his love for Lucía.