When we study aspects we use a degree of inexactitude. For example, we might say Moon is square Venus when the first is at 13 Pisces and the second at 15 Gemini. Orbs address the question of how wide is an allowable level of difference between the numbers; the orb is the size of the gap between actual and exact. There is no agreed definition of what is permissible, and we have to make up our own minds about how wide we choose them to be.
This applies as much to transits as it does to birth charts, so the exact conjunction of say Saturn and Pluto, which occurs on 12 January 2020 may not be the moment when its influence shows - it almost certainly will not be. In fact it could be months earlier or later, and will very likely be obscure until astrological historians look back from a future date.
In a birth chart reading we all have our own methods and principles, so each of us decides how big an orb to have, which affects the lines printed on the chart, and how much importance to give each of them.
Personally, I use tight orbs...something like this:
Conjunction 10 degrees; Opposition 8; Square 5; Trine 4; Sextile 3; Semi-square 2; Quincunx 1.5 and yet other astrologers, even the big names, vary up to quite large ones. It depends on several factors I would say: