I feel like everyone uses the term IQ but most people including me do not really understand what it is measuring at a fundamental level. Is IQ the same thing as intelligence or is it just one way of estimating it? And what is the actual difference between having a high IQ and just being good at IQ tests? I want to understand the concept properly before I go down the rabbit hole of reading about it.
IQ is our best available measurement of general mental ability, not the ability itself 
Well designed tests sample enough different cognitive demands that the shared variance across all of them captures something real and stable. That underlying factor, called g, predicts academic achievement, job performance, health, and life expectancy better than almost any other single measure in social science. The score is imperfect but it is pointing at something genuinely meaningful 
The simplest way to think about it is that intelligence is the thing and IQ is the ruler we use to measure it 
The ruler is imperfect but it is the best one we have and it is a lot better than most people assume when they first encounter the concept.