An organelle is a specialised structure inside a eukaryotic cell dedicated to one specific job, the mitochondria generating the cell's usable chemical energy, the nucleus storing and protecting the cell's genetic material, the ribosomes building proteins from genetic instructions. Rather than one undivided cellular interior handling every one of these genuinely different jobs in the same shared space, dividing labour between distinct, specialised compartments is exactly what lets a complex eukaryotic cell run several demanding processes simultaneously without them interfering with each other.
Each organelle's specific structure is built directly around its specific job
Mitochondria have a highly folded inner membrane specifically to maximise surface area for the chemical reactions that generate usable energy, the nucleus has a double membrane with controlled pores specifically to protect genetic material while still allowing necessary molecules through, and ribosomes are built from RNA and protein specifically shaped to read genetic instructions and assemble amino acids into a protein chain. In each case, an organelle's specific physical structure isn't incidental; it's built directly around efficiently performing that organelle's own particular specialised function.
Physical separation prevents different cellular processes from interfering with each other
Keeping genuinely different cellular processes physically separated inside their own dedicated organelles prevents them from interfering with each other chemically, energy-generating reactions that would otherwise disrupt delicate genetic material stay contained inside the mitochondria rather than running loose throughout the whole cell, and genetic material stays protected inside the nucleus rather than exposed to processes elsewhere that might damage it. This physical compartmentalisation is exactly what lets a single eukaryotic cell run dozens of genuinely different, sometimes chemically incompatible processes at the same time, each one contained within the specific organelle actually built to house it.
What we're still unsure about
That organelles divide a eukaryotic cell's labour into physically separated, specialised compartments, each with a structure built around its specific job, is extremely well established, thoroughly confirmed cell biology verified through decades of microscopy and biochemical study. What's more genuinely an active area of ongoing research is exactly how tightly different organelles' activities are actually coordinated and communicated between each other in real time, since a cell's organelles aren't fully sealed off from one another, they exchange materials and signals constantly, and researchers continue to work out the detailed molecular mechanisms coordinating that inter-organelle communication, rather than that fuller regulatory picture being completely mapped out.
This sits inside Organelle Structure & Function, one of seven topics in Cell Biology, one of six domains in Biology, one of seventeen subjects the app can quiz you on.