Bass is the low pitched sound content of what you're listening to. Treble is the high pitched content. Literally if its something that sounds high pitched to you, that's treble. Anything in between the low pitched and high pitched content is called 'midrange'.
A bass drum goes "boom". A bass guitar goes "bump, bump, bump". Those all sound like low pitched descriptions, right?
A cymbal goes "crashhhhhhhhh" and a wind chime goes "twinkle, twinkle". Both high pitched sounds. When you adjust bass or treble, you're adjusting how loud the elements in those ranges of the audio spectrum are in the overall mix of the sounds you're hearing in your music/media.
If the "crashhhhhhhh" is too loud and hurts your ears, turn the treble down. Similarly if the "boom" is overwhelming, turn the bass down. Just adjust them to your tastes and that's all you need to know.