The planets in your chart do not live in isolation: they converse with each other. Those conversations are called aspects, and they are the angles they form in the circle of the zodiac. An aspect can be a fluid alliance or a tension that pushes. They are the dynamic soul of the chart.
The conjunction (0°): fusion
Two planets together act as one, amplifying each other. It is the most intense energy: they can reinforce each other wonderfully or step on each other. It depends on which planets they are.
The opposition (180°): mirror tension
Two planets facing each other. You pull from one side and the other, and the challenge is to integrate them instead of oscillating between them. Often it is lived out projected onto relationships, until it is recognized as your own.
The square (90°): friction that drives
The aspect that pushes the most. It generates friction, discomfort, and for that very reason, movement. Squares are the engine of growth: life forces you to resolve them, and in that work your strength appears.
The trine (120°) and the sextile (60°): harmony
The trine is natural talent, something that flows without effort (so easy that sometimes you do not even make use of it). The sextile is opportunity: a gift that activates if you put a little will into it. They are the gentle alliances of the chart.
The harmonies give you gifts; the tensions give you character. A chart without friction would be comfortable and flat.
A key detail: the more exact the angle (the smaller the 'orb'), the more strongly the aspect is felt. A square within less than a degree can be a central theme of your life; one at six degrees, barely a murmur in the background.