@Somnium tramming is making sure that the x-axis is parallel with the plane of the bed (note not necessarily the bed itself, because that can woble a bit) So typically the Y-modules. Making sure those are well aligned. If you search the forum you can find some pictures on how you can do it, this is the first one I found: Milling the cnc wasteboard flat (or so I thought): mis-aligned z-axis modules - #16 by brent113
I don’t have 1-2-3 blocks as @brent113 is using in that post, but I use cans of soup or something like that. Probably not as precise as 1-2-3 blocks, but still rather adequate. And to validate I pull them out of their position and swap their position, they should slide under nicely again. (that way you also validate both cans are the same height.
Works good enough for me.
If you don’t have a problem with it, you usually don’t really have to worry about it too much. You would see it if your bed has a slope along the x-axis. I encountered it the first time after violently crashing the cnc module in the bed that caused it to get unaligned. (and mess up my cnc project at the time)