The Exposure Triangle is a common way of talking about the relationship between the three variables that determine the exposure of a photograph. These three variables are Aperture, Shutter Speed and ISO. You need to have all three balanced to get the best outcome for your photo. Generally if you adjust one of these you need to make adjustments in the others as well to get the perfect shot.
See here for a quick guide to the exposure triangle
Knowing the exposure triangle is crucial to getting technically correct images, or can be the difference to an OK shot versus one that looks simply amazing. The b best way to get to grips with this is to set you camera to M (manual mode) and start experimenting with the 3 variables. Without going into too much math detail, doing this will help take your photos to a new level.