A conjunction happens when two planets sit at (or very near) the same degree of the zodiac — the tightest possible aspect, and the only one where the two energies genuinely fuse rather than relate to each other from a distance.
Whether a conjunction reads as harmonious or difficult depends entirely on which two planets are involved — Venus conjunct Jupiter blends easily, Mars conjunct Saturn can feel like pushing against a wall — but either way, the two planets' stories become inseparable from one another in the chart.